We're a Window$ free house; Linux is the daily driver on every single computer we have.
I have school age children. They have IT classes. I set up a machine with Mint, clean install, to serve has the school workhorse. Not one task assigned at school can not be done in the Linux box. My child came home a few worried a few times because the teacher disliked having a linux box in the room.
What happens is the teacher is terrified has they cannot load a single piece of software on that machine, as they do with all the other students, at will. The notion of explaining to all the other students they need to go to some site to download some program while my child just needs to fetch it (or already has it pre installed) from a secure repository is baffling. The knowledge that that humble and rather older machine can not be trivialy tampered with is mind disturbing.
At some point the teacher explained how to maintain the system (clean temp files and random junk Windows collects over time by just having programs installed and removed) and looked at my child and chidded that was something she could not do.
I taught my kid how to do basic system maintenance. Through the console. Like a boss. They upgraded the system while they colleagues were "busy" hunting down temp files.
I feel for the teacher, Windows is still the predominant OS that is used by businesses worldwide and it's unlikely to change any time soon. Ensuring the kids have some familiarity with it is important as when a lot of them go into the workplace their employer isn't going to give them a choice of OS to use. A number of schools in my country now provide kids with Windows laptops that can be managed through group policies. I can imagine the teacher feeling frustrated at times as their teaching material will be geared to Windows and may face challenges in being able to grade your kid.
It's great you have given your kid experience in using Linux and that should set them up really well to working in a Linux based environment. Hopefully one day other OS will be added to teacher's curriculum so that all kids have the opportunity to get hands on experience.
If anything more recently schools have been going the opposite direction. Moving away from Windows towards Chrome Books which is probably even worse from a being prepared for the future stand point.
I started using Chromebooks in elementary school and my school used them all the way through high school. I can confirm that the majority of kids probably have no idea how to do anything outside of chrome and the google suite of apps since that's where the majority of our digital literacy curriculum focused.
My country invested a good deal of money into this, a part of which went into providing computers to students and schools.
There still is a machine installed with this in the school where my kids go but the teacher has (alledegly) no knowledge on it and personally refused to even broach the existence of anything else but Windows. There is no formal curriculum for this class; it's mostly used as a lab for kids to do assignments from other classes while learning to use the internet and an office suite.
I offered to provide a test machine for the kids to play with and explore, even go to the school and talk about FOSS and Linux (not an expert by any measure) and the mere offer was met with distrust and discomfort.
And as a former windows fanboy - I loved XP to bits and pieces - learning to use a computer under linux makes things a lot easier when needing to use Windows. The thing does not have a steep learning curve, I'll give them that.
Until something goes wrong in windows, and unlike in a sane system, the logs are filled with gibberish, there's no embedded documentation, and the online help is either turn it off and on again or just reinstall.
Windows is still the predominant OS that is used by businesses worldwide and it's unlikely to change any time soon. Ensuring the kids have some familiarity with it is important as when a lot of them go into the workplace their employer isn't going to give them a choice of OS to use.
This is why it's unlikely to change any time soon. Ensuring part. Windows is OS of cheap workforce because it is OS taught in public schools. Ban windows in schools and in 20 years entire country will shift to Linux.
I've been working full time in software dev and hardware dev since the mid-1990s. Through that whole time I've worked almost exclusively on (in the early days) Sun workstations, AS/400, and HPUX machines. This eventually transitioned to Linux and macOS (once it became Unix based). Over the past 7-8 years, every company I've worked for (primarily in backend software and "big data") has actually heavily restricted Windows within the company. Most have required high level approval to have a Windows machine... you had to have a damn good business reason to run Windows as your primary OS.
Windows is definitely the leader in generic desktop work, but... there are pockets out there of Linux/macOS-only. And... given the strong shift to browser based everything... Windows has lost its shininess for all but the most specific applications - eg graphics editing in industry standard tooling like Photoshop.
Thankfully the school my kids go to doesn't really give a crap what you run at home on on their laptops they used for school work as long as the kids are able to to their assignments. Almost 100% of what they do is browser based interfaces anyway, so it doesn't matter what the underlying OS is. I've made a point of teaching my kids Linux, macOS, and Windows. They've both asked to run Linux on their personal PCs... it was, and remains their choice.
There's also the fact that later on if your kid wants to certain things, either as a hobby or just with their friends, they're SOL because they don't run on Linux and the FOSS alternatives are awful and would scare them away. Kid wants to play a game with Kernel level anti-cheat with their friends? Nope doesn't work with Linux, unless they want to risk getting banned. Want to try your hand at video making? Good luck using obscure software that may or may not spontaneously crash on you and getting cameras to talk to your computer properly. Get a new toy that talks to your computer? Ha ha nope in your dreams
Sure you might be able to fix those problems, but can your kid? Can your kid do these things by themself and foster a sense of understanding and mastery over Linux, or are they going to grow up thinking that they can't do anything on their own computer because they constantly have to call over their dad for help?
Growing up my house was a Linux household and the first thing I was taught how to do was how to dual-boot into windows because letting me play The Sims and have fun was a little more important than ideology wars
Hey as someone who kinda grew up in that scenario, I really reccomend you show your kid what a windows dual boot is
Your kid doesn't exist in a vacuum. They have friends and inevitably your kid's going to be in a situation where their friends are like "hey, want to play this game with us?" And they can't because it's got a kernel anti-cheat that doesn't work with Linux. They're going to try and get into a hobby, only to find that the software everyone uses doesn't work on Linux and the alternatives that do are badly maintained and frustrating to work with. They're going to encounter a programme they need for school that just straight up does not work on Linux.
Sure you might be able to find a work around to all these things but like, can your kid? Because I speak from experience when I say that feeling like you have to be constantly running to your dad every time something doesn't work doesn't foster a sense of mastery, it makes you feel like you can't do anything on your computer because you're too small and dumb.
The teacher probably isn't "afraid" of the Linux box, they're probably frustrated that they don't know what's going on and can't help if something goes wrong. The programmes they'll probably teach your kid aren't a perfect 1-to-1 match to their Linux alternatives and they'll be left sitting in the back confused and upset while everyone else is learning about stuff in word and excel that you can't do in libre Office. You're not going to be known as the cool hacker dad, you're going to be put in the same category as the crunchy mum who doesn't let their kid eat sugar and needlessly restricts something that's just so petty to the layman.