In 1996.
That meant Debian on desktops (in classrooms, 486s with boot floppies and 10MB LAN mounting root over NFS and then XDMCP'ing over to a bigger X server; in the offices and computer labs, Pentiums with local installations, still sharing /home over NFS).
StarOffice was used for everything. We learned Perl in Grade 9 and had our own ~username websites. I ran a LambdaMOO.
I didn't realize how extraordinary this was until I visited a school with locked-down Windows NT 4.0 workstations and began to understand how amazing of a perk I had by having a computer teacher who pushed for this, against the wishes of more conventional teachers who wanted the safe option.
FVWM95 was a godsend in this regard - we could fool some people into thinking they were just on a weird variety of Windows.
I credit my career to this choice and wish I could find the teacher to thank him, but he has a John Smith tier name making him impossible to track down.
TED Talk: https://www.youtube.com/watch?v=f8Co37GO2Fc&app=desktop
RedHat Video: https://youtu.be/Nj3dGK3c4nY
The IT Director there has also written a book: https://www.amazon.com/Open-Schoolhouse-Building-Technology-...
[Full Disclosure: He and I are good friends.]
I spoke with the IT manager of the school and he's a real fan of open-source. He gave a talk at a open-source convention about it (in german)[0]. In this video he talks about how he convinced the local government to budget this, how it saved cost and much more.
I think this is a great example that this can work and more schools should try this out. From what I heard, the teachers didn't have too many problems and my classmats were quick to adopt without many problems. If I'm honest, I experience more problems with the Microsoft stack in my current school.
[0]: https://www.youtube.com/watch?v=fY47YY5BIGc