That would require legal changes. At the moment many schools are legally required to censor students.

This is also in a country which still doesn't teach comprehensive sex ed' in many places and for which many adults don't want kids to have access to that information.

HTTPS proxies work less well today than they did even five years ago. Certificate pinning and other security improvements have broken a lot of things (by design).

So the question is: What is more important, allowing HTTPS proxies, or stopping governments with a CA from MitM-ing traffic (e.g. Iran, China, etc).

A nice compromise might be to inform users that they're being MitM-ed by an installed CA, but only once and subtly so.

I'm pretty sure a DPI-based filtering can be purchased as an off-the-self service.

And that's one of the core reasons I dislike HTTPS everywhere. It will lead to organizations like schools installing their own root certificates to MITM traffic, lessening the security of those things where encryption is the most important, like online banking.

Plenty of vendors are ready to sell turnkey censorware to them. It's already a thing.

Nope. Pretty much 100% of schools in the U.S. (with internet access) operate a transparent or explicit HTTP(s) proxy as the only way out to the internet from student-accessible vlans. Websense is probably the most popular commercial solution. My school district ran a Squid extension called Squidguard for a long time before switching to the filtering solution bundled with its new wireless controller system.

Implementing web filtering is a condition of receiving federal subsidies; they take it seriously. A school with only a handful of PCs might use a host-based solution but proxy-as-the-only-way-out network design is very, very common.