I've read a few comments about card tricks in schools, so I thought I'd contribute to this discussion. For some reason the attitude amonst many is that to do magic in school you have to be a Blaine character, a miracle worker who wanders around floating objects and doing 'weird' things with objects all over the place. I'm very uncomfortable about this idea, because in a school setting one cannot act the magician 24/7, and if they do they will be written off as a bit of a weirdo. If magic is a hobby of yours and you want to demonstrate it to entertain your friends, while making it clear that this is still you and you are demonstrating something you have studied; that is fine. Card magic is ok. Remember: people like magic with cards. It's card tricks some people dislike.
Audience control is of course, key, and that starts with attitude. Make it clear that you are trying to entertain them, not present them with a puzzle. You absolutely have to stress that. In a school, you cannot assume your audience knows that. You have to be firm about repetition, just explain that you are aware that more people showed up now who didn't see it the first time, but that you cannot repeat an effect as a matter of principle, because it will no longer be magic. Keep a distance between yourself and the audience, so there is no oppertunity for hecklers to grab anything. If you are doing a card trick and someone grabs the deck; "What was the point in that?" Stop. Make it clear you won't work under those conditions. "Now find it!" No. Why should I? I'm just trying to entertain you guys, I'm not trying to prove anything. "You can't can you?" If I could, why would I? It is very boring to just find a card, without context or presentation. Anyone can do that.
I don't think having an out to meet their challenge is at all necessarily, as you should not be rising to a challenge. That's not what you're supposed to be doing. It is so very easy to have a heckler clearly branded a spoilsport who is missing the point, if you present yourself as an entertainer. Banter, make jokes, be friendly. Smile, but not smugly. Don't perform to those who won't appreciate it, and don't rise to challenges if they gatecrash you.
Performing the trick should be effortless anyway, so focus on your audience. It is all too easy to get absorbed in your own trick, and find yourself snapped out of it by a heckler who is fed up with your self-indulgence. By focusing on your audience, you can ensure that everyone is enjoying the magic. Make sure people can see clearly, and draw attention to things that may seem obvious just so it is clear what has occurred. These moments of recap are a great time to do your dirty work, of course.
Afterwards, they will want to talk about methods. That's normal. Steer them away from any real methods if you can. Sometimes to a particularly inventive (or indeed accurate) suggestion I will respond "That's quite an interesting idea. I might try that." Generally it is best of course to try and steer people's conversation away from the method to the effect. Five minutes later they won't remember your exact moves, but if you get it right, they will remember what you did. This is very good for you.
Wow dude, this is good stuff. You don't mind if i print your post out and frame it, do you?