That's true, so you alternate: do some preparation, then some fun stuff. As the date of the competition approaches, intensify the preparation.
Do you have any kind of funding to offer prizes, etc., or a system to give members in school rewards (for that you'll have to work with your HS's school administration)? If so, after the competition have a party, and/or give prizes to those who place. It doesn't have to be anything big; a little recognition goes a long way.