If you tell your brother that maybe the other students are just playing with him/Maybe they are just kidding around/Maybe they are only joking like friends joke with each other, you are saying that you think that whenever they do it, they do it not seriously, but in a friendly way. What you are saying is understood in the present and it means all the time when that happens, habitually. The problem is that what they say to him is hurting him, even if they are not intending to hurt him.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.