In a class discussion, building on someone else’s point counts as much as making your own.

Discussions are not a queue of unrelated speeches, though they often turn into one, with everybody waiting to deliver the thing they prepared. Taking somebody’s idea further requires that you were listening and that you can see where it goes, which is harder than having your own point ready. It also makes the conversation actually move.

Next time, try opening with a reference to whoever spoke before you and then add the bit they did not get to. Name them, since people remember being built on. If you disagree, the same move works: say what is right about it first, then where it breaks.