Skip folders and files during `grep`

I have a folder (/test) that contains several subfolder and files (at various level). Inside /test, I want to iterate only text files containing a particular string ($string), ignoring files and entire directories specified in a file ($to_skip) by path. This "exclusion list" contains both files and folder paths as shown below:

/test/ex1/fileA
/test/bob/ex1/fileB
/test/ex1/subfolder
/test/jerry/ex2

that is, one path per line. What I've done follows, but it didn't worked (the various options are required for other reasons):

grep -nriFlI "$string" "/test" | grep -vFf "$to_skip" > "$out"

The 1st grep actually give me the correct list of paths (any text files containing $string), but the 2nd grep doesn't filter as expected since $out contains all the items produced by the 1st grep except the occurrences corresponding the last pattern.

The strange thing is that it works correctly only for the last pattern. For example, if /test, among the others data, has the following files/folders

/test/jerry/ex2/f1
/test/jerry/ex2/f2
/test/jerry/ex2/foo/f3
/test/jerry/ex2/bar

and /test/jerry/ex2/ is the last pattern specified in $to_skip, the paths above (and in general any data under /test/jerry/ex2/) are being excluded correctly!

How can I achieve my target?

Many thanks!

As suggested by Dominique, putting -prune to find command will be
a common method to exclude specified dirs/files out of the search result.
Would you please try the following:

#!/bin/bash

# create the option to "find" such as "-path /test/foo -o -path /test/bar -o ..."
while IFS= read -r f; do
    (( ${#opts[@]} > 0 )) && opts+=("-o")       # 2nd argument and thereafter
    opts+=("-path" "$f")
done < "$to_skip"

find /test \( "${opts[@]}" \) -prune -o -type f -exec grep -niFlI "$string" '{}' +

• The array opts contains the exclusion list created from "$to_skip"
• The pair of \( .. \) is needed for the precedence.
• The -r option to grep is dropped because the argument list is already
  expanded as recursively traversed files.