Quorum

I think this is correct.   I have seen it used more often in business or government situations but this seems to fit the definition.

It has something to do with representation, voting, decision making, like that. The class'd better be very mature and active, because sarcasm threatens to suggest itself here.

So, what's another way to say this, using another word with the same meaning, I mean?

You could say "minimum number."   But, unlike Robert B,I don't think there would be any problem at all using "quorum."   It means "minimum number of people required to do the business at hand."   I only mentioned, previously, that it is usually found in the context of business or government, but it fits in this situation as well.