Don't get so ahead of yourself. Even people in relative poverty, in many cases, have new(er) iPhones, and HEIC is default on my SE, the budget iPhone model from 2016. In high schools you're alienated if you don't have the "blue bubbles"* - although it's stupid, if you're in high school in 2020 you probably need an iPhone, and you can get older models for a relatively small price (my SE was <$100). If anything, the ones with the most privilege will have enough technical training to know to make their phones default to JPEG rather than HEIC.
[1] - Source: I currently go to high school.