Illegal to fly about 14000 feet?

If you got in a Cessna 152 or 172 and you went above 15000 feet without having bottled oxygen, can you get in trouble? Would they detect you on radar and then find out whether you had oxygen on board? i've always wanted to see if I could get one of those little airplanes at that altitude. but I don't want to get in trouble with the FAA...