It is to their detriment when they find them uncomfortable and a distraction from learning. I know I couldn’t concentrate and produce my best efforts if I were in a mask all day, and I don’t really find them a problem to wear.
It’s plain daft to compare shops, busses and cinemas to school. People have a choice about whether to go to shops or the cinema. None of those things are compulsory. Kids don’t have a choice about going to school, it’s a legal requirement. And no film or shopping trip or bus journey to school takes at least 6 hours.
If they want to wear them for their own protection, then fine. Secondary teachers should be allowed to do the same. But like it or not, the risk for teenagers is tiny and they have already sacrificed enough for the sake of other people.