I would say something, without exposing this very nice trick:
1) Not true it's so easy to setup...because you can't setup in front of your mates, while they are looking at you (you need some misdirection)
2) the setup needs to be accurate, or your card wont float so high

but to reach the perfect setup just pratice...not hard to reach a near-perfect setup!
3)you can pass a card (or a pen, etc) above and under your card, to show there's no thread that keeps the card from above, and nothing that makes the card floating from the bottom of the card (between card and your hand i mean), that's very effective!
SUGGESTION: using cards is sometimes boring for people, something different from a card trick may be preferred..so i suggest to borrow a bill, or a businnes card, and perform the same trick with that kinda items!
borrowing items and create magis with THEIR items is always the best thing in my opinion!
hooe that helps

and sorry for my english!