At least Firefox supports a way for LANs to disallow DoH usage via regular-DNS responses: https://kb.isc.org/docs/using-response-policy-zones-to-disab...

Possibly, Chrome/Edge support the same or similiar mechanisms, but I haven't checked recently.

Alternatively, your school uses a HTTPS middlebox that knows how to distinguish DoH requests from regular SSL traffic to 1.1.1.1.

In any case, this is most likely not an 'attack' on you, and characterizing it as such is unlikely to lead to fruitful discussions around the policy.