Reduce

Think twice the activity you are going to do and if it is necessary to hand out some worksheets. Always think if the information you are going to give is useful or not, you can show it to your students by other means: computer, projector, reading... Try to print photocopies on double side.
Challenge pupils to bring waste-free packed lunches into school, by avoiding non-reusable and non-recyclable packaging.
Use natural light as long as you can and avoid using electricity if there is enough light outside.
If the heater is too hot there is no need to open the window and loose energy, regulate it instead and change it to a pleasant temperature. Switch everything off after classes and before holidays, weekends and mid-term breaks.
Teach your students about renewable energy sources and about how they can implement them on their daily life.
If you are not going to use the computer, the projector or the digital board, you have to ensure there are no electronic tool switched on.
Choose each week a different class assistant that would be in charge of switching off the lights of the class when the class goes to the playground.
Create bathroom rules in order that students do not waste water, they turn the tap down, use the necessary toilet paper, not more and switch off the lights when finished.