I'll throw my two cents in since I am both a teacher and a magician. I only allow my 4th grade students to bring in cards on Fridays if they are going to demonstrate a trick for the class with the 10 minutes of down time we have before lunch. Other than that, absolutely not.
As for allowing students to flourish during a class...I would definitely have to say no. How is that any different than allowing a student to doodle or draw while I am instructing? I do not allow anything to go on while we are working and learning.
Cards can be distracting for other students who already have a hard enough time trying to focus on the task at hand.
When I was in school ( 12 years ago ) cards were not allowed in our high school at all. I think the big issue was the desire to stop gambling. Now, that being said, are students really doing THAT much gambling? No, kids are just being kids playing games with their friends, showing tricks, etc.
I understand the urge for you younger guys to show off your new material and try it out to see the reactions you are going to get, but I also understand that from a teacher's standpoint, the cards are probably better left at home.
For the guys that say that they are not going to listen to your teachers and just do it anyway...you crack me up. Have a little respect for the adult who is trying to provide you with an education. That is one thing that I see lacking from today's younger generation. I am not even that old, (30) and I see how middle and high school kids talk back to their teachers and even their parents. My old man would have cracked me in the mouth for disrespecting or not listening to an adult.
I hope this doesn't stir up too many emotions but it might be food for thought for some of you.