As kids get older they are going to look at each other's bodies, THAT's normal. And think "ooh ooh a girl's bra exciting!" or "mmm I like his arse" or whatever.
It is subjecting other people to a degrading, humiliating commentary about their bodies and underwear, in which you blame them for your thoughts about those things, that is unacceptable. It's always unacceptable but the problem it is it is mainly boys who grow up thinking it is OK and normal to do this to girls. And no wonder when the same attitudes are often spouted by parents and teachers.
Having this attitude is linked to much worse harassment and abuse and assault because the attitude is that girls deserve to suffer, feel responsible, feel humiliated, and be dominated. If that same mindset continues to the point where the boy becomes a male student alone in a room with a female one, or an entitled, dominant man in a relationship with a woman, that's why we have the problems with sexism, assault, rape and general misogyny that we do. That's why women end up starting desperate threads about being pressured and coerced into sex by their OH. Or bullied, or emotionally abused, or just left to be responsible for everything. Because society tends to bring many boys up to be entitled, domineering, aggressive, woman-hating twats.