We checked our BMS system after looking at the report from your selves highlighting the holiday and weekend usage and very high percentage increase from last year. In consultation with our engineer we found that with our system if a room stat with a sensor was turned to summer mode, ie no heating as the room was hot most of the time during school day, the system will be activated to turn on heating. When sensing a room temp below 15c or outside temp of 2c to ensure the fabric of the building doesn't cool down too much, it will ask for extra heating  even during weekends or holidays. Therefore,with this one class being set to summer mode ie not being asked for heat, as it would sense the temp would be 15c or below, it would then turn the heating for the whole school on in an effort to try and heat this particular room but never achieving it due to stat being set to summer mode.