I'm going to critique someone's instagram posts even if they're factually incorrect and could cause harm
Why would you want to harm someone like that?
He was being such an asshole to me, he once mocked me by my school grades.
Mocking someone back that has mocked you will not fix the problem. It can create a cycle of animosity; there is no reason for it.
What he did deserves this critique. It will be honest opinion for a piece of shit.
He may deserve it in theory for being mean, but don't you think that he will retaliate if you do? You can inform a teacher if someone is bothering you, do not take it upon yourself.