Review your energy use charts for gas. Do you notice a large daily variation in your school's gas consumption during the winter months like the chart below?
This could be due to your heating system responding to changes in outside temperature.
To check this, look at historical weather data for your area. Your school may have its own weather station or you could use http://www.paulwilman.com/previous_data.php
which is a great website showing real time and historical weather data for Bath.
Compare days with high gas consumption, does this correspond with low outdoor temperatures? Is the weather warmer on days with less gas consumption? If so, it demonstrates your school has good thermostatic control of its heating system.
If the variations in gas consumption don't correspond to outside temperature changes, can you investigate what else might be causing the change in consumption. You may find that energy is being wasted on high consumption days, due to doors and windows being left open for extended periods. This could be due to poor thermostatic control in individual rooms, or a poorly located main boiler thermostat. If your main boiler thermostat is located in a poorly insulated room with few radiators such as the school hall it may never get up to the set temperature causing the heating controller to run the boiler constantly which causes better insulated classrooms to overheat.
You may find that the programming of your boiler controller could be improved. Ask your caretaker to check whether the boiler is coming on unnecessarily, and whether you could better locate your heating thermostat.