And that´s why:
With a negative sentence in which you use "did not/didn´t
" it means that the (main) verb doesn´t conjugate as we learn in basic English (when we learn verb to be/to have, for example). The verb don´t go the past form
The "did not/didn´t
" simply "kills" the conjugation, and then the verb works in its infinitive form (present infinitive form, some people would say).
Say, by way of examples: take
continues as take
/work as work/give as give. And so on...
Back to your initial sentence then:
"I'm sure they didn't took English in high school." The verb is not correctly used, it should be "I'm sure they didn't take English in high school."