> Just log off of Instagram / TikTok / wherever.
A large number of youth will do that, but there is also a reason why children and youth are considered a vulnerable population.
I cannot speak too much on youth since most of my experience is with children. While some of them are more than happy to navigate their social lives by choosing who their friends are and ignoring those who cause them grief, where possible (as you suggested, there is the physical proximity in schools to complicate things), a great many more want to belong. They want to belong even where they are unwelcome and the unwelcomeness is manifested as bullying. What little I've seen of teens suggests that much the same does happen, only the bullying is meaner.
And that only addresses bullying. It does not address content directed towards adults, much of which is perfectly fine for younger people to know but they should also learn about it under the guidance of an adult so they don't come to think it is acceptable or the norm. It also fails to address the manipulative nature of some sites, something that they probably haven't learned to recognize never mind how to handle.
Yes, in a sense, you can turn off the problems and a lot of the problems will just go away. I'm not going to say they will disappear completely. Remember, social media has a larger audience and the others in that audience may bring it back to real life. Also remember that the people we are discussing don't always have the wisdom or experience to turn it off. Heck, a lot of adults don't have that wisdom or experience (but, at least for adults, we can claim they ought to know better).