If you really need it for school, then you should be able to get a free copy from your school. The requirement for this typically is simply being enrolled in an IT related course. If not, you can still get it cheap for £/$/€30. Just google "Microsoft Ultimate Steal Deal", provided that you have a valid academic email address.
If you don't really need it for school (i.e. if you're in middle or high school) then you shouldn't say so. Ignoring that last comment, there are many ways to get things free with the minimum of web-awareness. Advising specifically how in this case would be against the rules (as already shown).
At the very least, we can tell you that legit keys will always be legit so long as you're not using the same key across multiple computers. You are allowed to uninstall Windows from one computer and use that key on another computer (seen as a licence transfer), so long as you don't reinstall Windows on the first computer and use the same key again (at least, without uninstalling it from the second computer first).
As for using illigitimate methods, try as much as possible to keep them completely seperate from your legit keys. Many legit keys get blacklisted due to careless use of cracks, keygens, loaders and other such tools of the pirating world.