Minecraft Java can be run on any Windows machine, and can be bought and downloaded here. In particular, it works with machines pre-Windows 10, which may describe your school computers.
Minecraft 10/Education Edition
These versions are very similar, but the Education Edition offers more support for the teacher, allowing them to direct activities and limit pupil experience where necessary: the Minecraft 10 version is for personal use. These editions both work on Windows 10 and MacOS machines.
Free Trial of Minecraft Education Edition
If your school doesn’t have funds to purchase either of these versions, you can install Minecraft Education Edition and use its free trials to explore Saint Lauretia (25 for teachers, 10 per pupil).
Go to this page, log in and refresh the page, and then install the ‘Trial’ version.
If you are unsure which version of Minecraft you need, or how to install it, it is recommended you ask your school IT administrators for help.