Some schools already started to block Firefox from being installed because it was being used as a "VPN" by some elementary students to access porn - at school. Guess what this VPN was? Just DNS over HTTPS.
There is a fine line between your ISP and local network operators that NEED to apply some security policies to their traffic. Even Google offers "Safe Search" for schools and libraries that removes porn content.
Unfortunately, on our school network, we also allow BYOD, so we will have to have some strict rules to block DoH, the same way we block proxies and vpns.