A similar issue, Google in schools, was discussed in this thread.
Essentially, you can compartmentalize by using Microsoft Office (as absolutely necessary) solely for school activities. This may safeguard against your personal life being uploaded to the cloud. Compartmentalization has limitations however, so think of it as just harm reduction. It is difficult if not impossible to compartmentalize your identity, thoughts, writing style etc into separate compartments, and the cloud backups would get a hold of that.
That being said, unless/until you, other students, parents, teachers etc who think alike convince your school to drop Microsoft products for LibreOffice or something else, compartmentalization and exposure minimization may be the best you can do. You could try refusing to use Microsoft outright (if it is safe for you to do that), make the school very aware you are unhappy with Microsoft in schools, or change schools.