Most school boilers have frost protection settings. They can be configured to turn the heating on at a low level to protect hot and cold water pipes against frost damage when the temperature is below 4C outside and the school is unoccupied, for example at weekends and during holidays.
It is much more efficient to make sure your frost protection is working than to leave your heating on over weekends and during holidays if you are concerned about frost damage. Frost protection will typically only come on when the outside temperature is below 4C and the inside temperature has dropped below 8C, rather than leaving your heating system on at 20C regardless of the outside weather during the holidays. As a result of global warming, it is less common for the average daily temperature to drop below 4C.
If you do have frost protection enabled, make sure it is configured efficiently:
it only needs to come on if the outside temperature falls below 4C
and the inside temperature falls below 8C (a conservative value to avoid areas of the school away from the thermostat falling below 4C)