The biggest cause of teen suicide is essentially feeling rejected in some way by your peers: being bullied, a breakup, "mean girl" switches from former friends, etc. When schools closed, the immediate pressure of any of these negative social interactions went to the background (or at least simmered on social media). When schools reopened, kids tried to figure out where they fit back in the "totem pole" of adolescent social hierarchy, and the rise in suicides is a result of kids not figuring out where they fit in when they went back.