You just don't have the skill bro, mentally ill kid is like the weakest link. How did you get the blame, i literally still remember that time I pick on a mentally ill kid he got mad and punch me, a teacher then ask about the fight and I just said "oh it was just a misunderstanding you know how he is" and I pretty much got scot free.
Rather than blaming the mentally ill for being a retard just said he misunderstand something and suddenly hit you 10/10 you wouldn't get any blame.