If someone asks you:
“Do high schools not teach students to dress properly to school?”
If I want to tell the person that high schools do tell students to dress properly to school, do I say yes or a no?
You say “yes”. If you want to say they don’t, you say “no”.
In some cases, this type of question is a rhetorical one and it’s best not to answer at all. The above question can and often are used to “politely” admonish the one being questioned. The questioner really means to say that the students lack the education or upbringing to properly dress for school. I’d either keep quiet or give the questioner the finger.