It's a classic damned-if-you-do, damned-if-you-don't for young women. Either you participate in selfie culture, making you a "slut," or you abstain, making you a prude. It's a game where whatever choice you make, you lose. The only answer is not to play.
From the moral high ground, they can damn a girl for visual promiscuity, yet enjoy the spectacle at the same time, both with the same misogynistic motives: I like your form but I'm able to scorn you. You're what I want but you're less than me. Girls try to conform to this ''ideal'' stereotype in their photos and these boys sarcastically comment, ''Nice personality'' - really implying that the cleavage is their only attribute. Yet they also click the ''like'' button. The boy who mocks a girl showing her cleavage is in fact the same boy who craves sexual opportunities with her.
A common adult reaction to social media is to restrict things, as if that could ever be possible. You can't force kids to be nice. The real problem isn't something tangible like sexting or bullying, which adults focus on in patronising and unimaginative ways. The real problem relates to conformity. Kids are compelled to act the stereotype, because those who opt out commit themselves to social leprosy. Social media doesn't need adult control. What we need is some good taste.
UPDATE: Derek pointed out in the comments that "Year 11" means grade level, not age, so I've corrected the post.