Irregular use, fixed attendance
The instinct is to clean a room after it has been used, which sounds sensible and works poorly in practice. Volunteer sessions move, they are cancelled, and additional ones appear before a school event. An attendance that tracks them will be missed as often as it is delivered, and nobody will notice because the room's condition is nobody's particular concern. By the end of a term the room has usually not been properly attended at all.
A fixed attendance at a stated time is more reliable. It happens whether or not the room was used, it can be checked, and over a term it produces a consistent condition. Where a school wants extra attention after a particularly busy period, that can be requested as an additional visit rather than by trying to make the routine schedule follow an unpredictable calendar. The routine stays fixed either way.
