If he's an art student, chances are he actually enjoys looking at art, so I wouldn't worry too much about it reminding him of school. I would suggest trying to find an art exhibition thats not the same old portraits etc, maybe something a bit more modern and fun, perhaps something participatory? Then you can bounce ideas off each other about what the art means etc.
Also, a group idea is good, but it can be awkward sometimes. If it's a group thing it's probably best to be something that you can split up into pairs and not be missed - otherwise you might end up having to keep talking with your friends rather than getting to know him better on a more intimate level.