Luckily some actually get it and use Linux in schools. But in my experience it's not common (in higher education chances are better).

I think the "it" some "get" is not seen as a necessary "it" for the purposes of education, and "Using closed stuff for education is a bad idea" is begging the question.

Linux is 25 years old now. The GPL is older than that. The ecosystem around them has had plenty of time to prove its worth as a superior solution for educational tools; it mostly wallows around in a sea of mutually-incompatible "maybe-we-could" and "what-if-we-tried" initiatives without the kind of clout behind them that make institutions believe the solution's providers will be around in five years and the institution won't be stuck owning 100% of the tool they don't really want to support. It turns out most educators and students would rather have a working solution guaranteed by someone with money in the game than a tool they can fully modify, evidence seems to suggest. The advantage to the MS ecosystem is that when it doesn't work, you can take it to multiple independent companies who will fix it for you.

It's quite clear that depending on some vendor who can dictate what to do for the education system is a bad idea. And entities like MS clearly have that power now to some degree (or strive to gain that power). So using open source in education field is a natural way to avoid that. As I said, some actually get it enough to act on it. I don't think anyone thinks that it's good to have that dependency in general, most just don't go anywhere further than realizing that it's not good.

Isn't all software opinion, regardless of origin?

But, there's a lot of value in having an open source and libre edition of anything which started out or is maintained as a closed-source, proprietary system.

Consider: Firefox versus IE, Linux versus OSX or Windows, or LibreOffice versus Microsoft Office.

With Minecraft, there is no opportunity for developers to really get on-board to build stuff with them. The source is closed and Mojang is AWOL when it comes to the developer API.

Minetest, on the other hand, is LGPL and is freely available.