@banjoanjoThere's no rule against being in more than one more club. I just believe that you'd need to be able to juggle it as in real life.
The strictness of a clubs attendance would be both dependant on the the club manager and the school.
For something like Student Council, you'd most likely have to report in for every meetup, whilst drama would probably be more lenient. Unless you were rehearsing a play that'll be shown to the public or something along those lines.
You could probably just join a club for the fun and be excluded from serious activities
ie; you enjoy playing basketball but are not on the team.
You enjoy helping the drama kids, but don't actually perform live. (For example, Jumani likes to watch the Drama people practice)
It's all determined by the club and the seriousness of their NPC or Player, Manager
Edit: Look at me, pretending to be a GM. These are all just suggestions btw, there's nothing concrete to them, so it's all up to us when you look at it.