How about running a whole school lights switch off competition whereby each class is allocated their own classroom and another area of responsibility in the school (eg toilets, corridors, halls, offices etc)? Allocate each class 100 points at the start of the competition. Monitor lights daily and if lights in their particular zone or classroom are left on then points are deducted from their score. The class with the most points at the end of the competition is the winner. Run an assembly to get people excited about taking part, and to outline the rules.
You could try running this for 2 weeks to begin with. If you find it works well, try running it for a longer time to establish good habits.