The Ubuntu developers have repeatedly said that the Netinstall (Mini) .iso image was a byproduct of their workflow of the time, not intended for widespread use, that it didn't provide the experience they wanted users to have, and that they wouldn't provide support for it.
(When I've seen Ubuntu Developers at work, I see them mostly use the Desktop and Server installers. Sometimes Ubuntu Base.)
So you were already using an unsupported install method.
The reason that the Netinstall .iso went away is that the installer development workflow changed. There's no longer that byproduct.
If a group of community members wants come together to restore the existence of the Mini.iso, they are completely welcome to take on that work. Ubuntu is open source and community-driven.
The Ubuntu 20.04 mini.iso still exists for a few more years.
Another option: Debian's mini.iso. You can edit the sources and dist-upgrade to use Ubuntu packages instead...which means you will be running Ubuntu.
The Ubuntu installer can be booted using PXE so you can customize your install. Or you can use a preseed file. Or a cloud-init file. There are many custom-install options.
Or you can just use Debian, regardless of what your school says. Ubuntu Developers strive to keep the differences between Ubuntu and Debian to a minimum (carrying patches costs resources!) Server users might not notice much difference.
And, finally, you can use Ubuntu Server. It is not bloated. It is fully-supported here, it is 100% real Ubuntu (to make your school happy), it's fast, it's easy to install, it's stable, and it's tested.
