I can't picture what you mean. "Put up your hands" sounds like a command from a robber to the person he's mugging.
Boys and girls, if you have a question, raise your hand. Don't just shout it out. -- Each person who has a question would raise one hand.
Boys and girls, we're going to get our blood flowing. Okay, everybody stand up. Right - now put your hands up in the air. (or: Raise your arms up over your head)