To answer the big question of why Koharu stopped going to school and socially withdrew, probably nothing specific. Oh sure there were events and things that happened, but as we saw there was stuff independent from Hozue that are unseen so it couldn't have been her and bullying lackies feels like a stretched cope since it would've been mentioned. It was likely a snowball of things, but ultimately(and what trips people up and why they can't fathom or lacth on to things) is that Koharu just stopped even trying to interact which the narrative has made EXPLICITLY CLEAR was the ultimate factor and problem.
The WHY is clearly insignificant and irrelevant and the main point I'm seeing here and what trips people up is that at the end of the day it's up to YOU to make these changes and most importantly THE EFFORT TO INTERACT AND CONNECT with people and stop making excuses for why you're not even bothering to do the bare minimum. If you've Magus of the Library it's the same things Sedona and Master Ganan tell Theo, it's scary and hard and probably hurts sometimes but if you don't power through and do something you're going to STAY alone and blaming "insert any number of excuses" is just you passing the buck for your lack of effort.