How do i make my string work inside of the switch in this program?

I am trying to make a code that does pick an int number or an char, inside on a string and display latter my options of age range if my input is an char bettewen A and D, and the class if i pick an age. I was inserting all in one big switch case but when i get to the "case 10:" simple the input dosent work, and i dont seen to make it work.
the code in question:

#include <stdio.h>

int main(){
    
    char resp[300];
    
    printf ("Inform the letter of the class\nOr the age of the student\n");
    scanf (" %s", &resp[300]);
    
    switch (resp[300]){
        
        case 'a':
        case 'A':
        printf ("\nFor students in the class A\nThe age range is between 4~5 years.");
        break;
        
        case 'b':
        case 'B':
        printf ("\nFor students in the class B\nThe age range is between 6~8 years.");
        break;
        
        case 'c':
        case 'C':
        printf ("\nFor students in the class C\nThe age range is between 9~10 years.");
        break;
        
        case '4' ... '5':
        printf ("\nFor the students in the age %s\nThe class will be A.", resp);   
        break;
        
        case '6' ... '8':
        printf ("\nFor the students in the age %s\nThe class will be B.", resp);
        break;
        
        case '9':
        case 10:
        printf ("\nFor the students in the age %s\nThe class will be C.", resp);
        break;
        
        default:
        printf ("\nWe are out of class\nFor students below 4 yo\nOr more than 10yo.");
        break;
        
    }
    return 0; 
}

When we get to the value 10 the code simple ignores this case and end the program, and the program does nott proper display the strings in the cases bettwen 4-9, is there any way to make any of this work with the switch?

I try to make an condition for to it to convert if the value was 10 this way:

if (resp[300] == "10"){
resp [300] = x;}

and switch the case "10": for case 'x':.
but this does not seen to make things work.

char resp[300];

This declares a char array of 300 elements. As a string needs to be zero-terminated, this can hold up to 299 characters.

scanf (" %s", &resp[300]);

This should read scanf( " %s", resp ). Or, better, scanf( " %299s", resp ), to avoid overflow.

You should ALWAYS check the return value of scanf() and its brethren. If input failed, resp would be non-initialized.

switch (resp[300])

This accesses the 301st element of resp (as numbering starts with 0). This element does not exist, the access is undefined behavior. If you would want the last element of the array, you'd use resp[299], but that isn't what you want either. If you'd want the last character of the string, you'd use resp[ strlen( resp ) - 1 ], but that isn't what you want either.

What you actually want is the first character, which is resp[0].

case '6' ... '8':

This does not work at all unless what you're using is some kind of extended language; C does not have a ... operator.

case 10:

This does not do what you think it does. Instead it compares a character's binary representation with the value 10. In ASCII, that would be the line feed.

All in all, the strong suggestion regarding user input is to use fgets() to read whole lines of input, and then parse them in memory (e.g. using strtol(), strcmp() or similar). Using *scanf() on potentially malformed user input is strongly discouraged; *scanf() is meant to read known good data, and its abilities to recover from error gracefully are limited. And switch is for distinct values. It does not work for ranges, and it is rather inadequate for working with strings in general.

getting some feedback from all you guys, and doing some research i made a rather simple solution to my problem in the following way:

#include <stdio.h>
#include <ctype.h>
int main() {
char resp[30];
int i;
printf ("Inform the letter of the class\nOr the age of the student\n");
scanf(" %s", resp);
for (i = 0; resp[i] != '\0'; i++) { 
if (!isalpha(resp[i])) { 
break; 
}
}
if (resp[i] != '\0') {//if the value of [i] is different of null the result must be in the range of a char.
int num = atoi(resp);//then i convert the string number to a int num
switch (num){
case 4:    
case 5:
printf ("\nFor the students in the age %i\nThe class will be A.", num);   
break;
case 6:
case 7:
case 8:
printf ("\nFor the students in the age %i\nThe class will be B.", num);
break;
case 9:
case 10:
printf ("\nFor the students in the age %i\nThe class will be C.", num);
break;
default:
printf ("\nWe are out of class\nFor students below 4 yo\nOr more than 10yo.");
break;
}
}
else {//if the value [i] is null the result must be a char.
switch (resp[0]){
case 'a':
case 'A':
printf ("\nFor students in the class A\nThe age range is between 4~5 years.");
break;
case 'b':
case 'B':
printf ("\nFor students in the class B\nThe age range is between 6~8 years.");
break;
case 'c':
case 'C':
printf ("\nFor students in the class C\nThe age range is between 9~10 years.");
break;
default:
printf ("\nInvalid Class.");
}
}
return 0;
}

One of the other answers has already listed and explained the errors in your code.

Therefore, in this answer, I will only show you the code for how I would solve the problem:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <stdbool.h>
//forward declarations
void get_line_from_user( const char prompt[], char buffer[], int buffer_size );
bool convert_string_to_long( char str[], long *num );
int main( void )
{
//repeat until input is valid
for (;;)
{
char input[300];
long num;
//get input from user
get_line_from_user(
"Please enter the letter of the class or the age of the student: ",
input, sizeof input
);
//determine whether the user entered an integer
if ( convert_string_to_long( input, &num ) )
{
switch( num )
{
case 4:
case 5:
printf( "For the students of the age %ld, the class will be A.", num );
break;
case 6:
case 7:
case 8:
printf( "For the students of the age %ld, the class will be B.", num );
break;
case 9:
case 10:
printf( "For the students of the age %ld, the class will be C.", num );
break;
default:
printf( "Invalid age! It must be between 4 and 10!\n" );
continue;
}
//input was ok, so break out of infinite loop
break;
}
else
{
if ( input[0] == '\0' )
{
//input is empty, so reprompt user for input without
//printing an error message
continue;
}
if ( input[1] != '\0' )
{
//make sure that there are no further characters
//after the first one, or that all superfluous
//characters are whitespace characters
for ( int i = 1; input[i] != '\0'; i++ )
{
if ( !isspace( (unsigned char)input[i] ) )
{
printf( "Input must be an integer or a single letter!\n" );
//we cannot use "continue" here, because
//that would go to the next iteration of the
//inner loop, but we want to instead go to
//the next iteration of the outer loop
goto continue_outer_loop;
}
}
//the following block is only intended as a goto
//target for jumping out of the nested loop
if ( false )
{
continue_outer_loop:
continue;
}
}
switch ( toupper( (unsigned char)input[0] ) )
{
case 'A':
printf ("For students in class A, the age range is between 4-5 years.");
break;
case 'B':
printf ("For students in class B, the age range is between 6-8 years.");
break;
case 'C':
printf ("For students in class C, the age range is between 9-10 years.");
break;
default:
printf( "Invalid letter!\n" );
continue;
}
}
//input was ok, so break out of the infinite loop
break;
}
return 0;
}
//This function will return true if the entire string was
//successfully converted to a long integer, otherwise it will
//return false.
bool convert_string_to_long( char str[], long *num )
{
long result;
char *p;
//attempt to convert string to number
result = strtol( str, &p, 10 );
if ( p == str )
{
return false;
}
//verify that there are no unconverted characters, or that if
//such characters do exist, that they are all whitespace
//characters
for ( ; *p != '\0'; p++ )
{
if ( !isspace( (unsigned char)*p ) )
{
return false;
}
}
//everything went ok, so pass the result
*num = result;
return true;
}
//This function will read exactly one line of input from the
//user. It will remove the newline character, if it exists. If
//the line is too long to fit in the buffer, then the function
//will automatically reprompt the user for input. On failure,
//the function will never return, but will print an error
//message and call "exit" instead.
void get_line_from_user( const char prompt[], char buffer[], int buffer_size )
{
for (;;) //infinite loop, equivalent to while(1)
{
char *p;
//prompt user for input
fputs( prompt, stdout );
//attempt to read one line of input
if ( fgets( buffer, buffer_size, stdin ) == NULL )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
//attempt to find newline character
p = strchr( buffer, '\n' );
//make sure that entire line was read in (i.e. that
//the buffer was not too small to store the entire line)
if ( p == NULL )
{
int c;
//a missing newline character is ok if the next
//character is a newline character or if we have
//reached end-of-file (for example if the input is
//being piped from a file or if the user enters
//end-of-file in the terminal itself)
if ( (c=getchar()) != '\n' && !feof(stdin) )
{
if ( ferror(stdin) )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
printf( "Input was too long to fit in buffer!\n" );
//discard remainder of line
do
{
c = getchar();
if ( ferror(stdin) )
{
printf( "Error reading from input!\n" );
exit( EXIT_FAILURE );
}
} while ( c != '\n' && c != EOF );
//reprompt user for input by restarting loop
continue;
}
}
else
{
//remove newline character by overwriting it with
//null character
*p = '\0';
}
//input was ok, so break out of loop
break;
}
}

This program has the following behavior:

Please enter the letter of the class or the age of the student: abc
Input must be an integer or a single letter!
Please enter the letter of the class or the age of the student: 6abc
Input must be an integer or a single letter!
Please enter the letter of the class or the age of the student: 3
Invalid age! It must be between 4 and 10!
Please enter the letter of the class or the age of the student: 4
For the students of the age 4, the class will be A.

Please enter the letter of the class or the age of the student: d
Invalid letter!
Please enter the letter of the class or the age of the student: b
For students in class B, the age range is between 6-8 years.

As you can see, I have designed the program in such a way that it will check whether the input is valid, and if not, it will automatically reprompt the user for new input.