My friend is so immature (kinda long soz)?

My advice is that you should just treat him like a kid. He really won't know what to do. After reading that, I'm sure it'll work. Also, warn him if he continues and never forgive him. Scare him and pretend to tell the teacher, or actually do it behind his back. Just tell the teacher to warn him and tell him that he/she will always be looking out for him. Hope this helps!