There is always a problem getting respect from people who knew you before you were a magician. I am assuming you are new to magic and your peers knew you before you could do these magical miracles. I have the same problems with some of my adult friends who knew me for decades before I started doing magic. They don't steal stuff, but they sometimes suspect that I am doing a "trick" (well, duh) and are not as easily impressed. Things got much better when I stopped pushing magic on them and I waited until my skill level improved. Now many come to me to see tricks. But there are some to whom I still do not show tricks. I do not even show my wife and daughter many tricks anymore. But you have a bigger problem. Kids, who are your age or older, are the hardest audience.
Here is something you can do. First, avoid the temptation to show everyone every trick you buy the day or two after you buy it. You have to control the audience, and that means do the trick only when you have one or two people you trust around--at least until you get better. Make people come to you wanting to see tricks but learn to say no, particularly if someone is there whom you do not trust.
Learn to do some ungaffed tricks. If you do a real trick with a dime and a penny (and Greg Wilson's On the Spot has a great dime and penny routine) it costs only eleven cents. And no one is likely to steal an ordinary deck of cards. Plus, when you do "real" tricks your confidence will improve, people will show you more respect, and if you do choose to do some gaffed tricks after you have shown your skill, people will assume that the gaffed trick is skill, too. They will hold you in awe.
Never make your tricks into a puzzle for the spectators and never try to look like you are in competition with them.
Kids can be mean, so just chalk this up to experience. And tell the teacher something was stolen.