Don't perform for friends, family or school mates.
Practice with a few magician friends, and go out and perform for total strangers.
I COMPLETELY AGREE!
I believe the problem you're having might be due to the fact that you're trying to sell the trick too hard. What I mean by that is if you make a serious claim like "you have so and so powers" then the crowd will be more curious and yet more suspicious,critical and even cynical. They will try to prove you wrong and show that you are a fake...So, instead I say "Do you have a dollar bill?", then I take it and levitate while they are watching in shock. I hardly call it before I do something, and give them the chance to watch it with a critical eye. I don't say " look, now I will levitate a dollar bill"...When you don't announce it, they don't expect it, and people generally are not as critical when they don't know what you'll do ahead of time...Surprise your audience, catch them off guard. When I got caught, it was ALWAYS when I announced it ahead of the effect...
In my experience it is harder to control people whom you interact on a daily basis closely. If someone knows you for quite a while, they seldom buy it when you claim one day suddenly that you have psychic or psychokinetic powers. So, don't decide to assume the role of Criss Angel or David Blaine one morning, and then get disappointed when they challenge you or catch you...I make sure they understand that I am doing this for fun. When you act casual, then your performance will be more comfortable and people won't be as anal...
It is easier to create a mysterious atmosphere with strangers since they don't know you. As many said here though, specs just don't know what exactly you're doing, but they are not idiots. Most of the time they know you're either faster than they can catch, or you're using something they can't figure out, and hardly they will think that you have supernatural powers. The fun part is they can't figure it out, so use it to your advantage, have fun. Now, I had some people believe that I could read their minds or have psychokinetic powers, but I didn't claim I had. Let your specs decide "what" you are, what matters is they just don't know what you're doing...If someone says you are a pyschic then go with it, or agree with it if a friend says "that's a nice trick!".
This was a long one, but I have been thinking about that a lot myself. So hope it helps.