It would be relatively easy for mischief-makers to switch off the heaters... The risk is limited because ... it should be possible for building managers to notice what is happening and manually override
If you really wanted to be malicious, you would curtail fresh air, rather than cut off heat. It would be harder to notice, and would give you an attack that didn't depend on cold weather.
And arguably the long-term consequences would be greater. The kids who got cold would bundle up and complain, leading to a (possibly energy-wasting) fix. The kids who got a low-grade headache and fatigue every afternoon might conclude "I'm just not cut out for learning."