We could give a class to them on the importance of conserving electricity. and also tell them how electricity is made so that they get more aware about the electricity. One more thing we could do is just show them the electricity bill which is increasing day by day due to their carelessness and we can tell them that if they go on wasting electricity then they will have to pay the money, so that they get scared and won't waste electricity here after. The leaders should make sure that the fans and lights are switched off before leaving the class. 
1 1 1