Walk around the school entrance or where he enters the school building like you're just bored. Don't make it seem obvious that you're strutting around the door, but make it look like you're hanging around, minding your own business (like text while you do this), or whatever you can come up with it. Then when he comes, look up from whatever you're doing as if you just realized he's there. Call him over by saying hi or approach him and don't forget to smile. Just be like "hey, what's up? It's been a while since we've talked!" or "how have you been since the last time we talked?" These are usual conversation starters and the reason why they are so cliche is because they work. Then go on asking him about what's going on in his life, comment on his clothes, talk about that weird teacher/homework assignment, or anything that you both can relate to.
Hope this helps and good luck!