If I were to buy a Sketchup Pro 2020 educational license, would I be able to use Sketchup Pro 2017 with the same license?
No. SketchUp licenses are version specific.
This could be an issue with wording.
If you mean can you still run SU 2017 after you install 2020, the answer is yes - just leave it as is, don’t uninstall it or try to apply the 2020 license to it. The licenses are version-specific and having a new version license does not invalidate the old one.