Why does Apple Music say that my student email is in use when trying to switch to a student account?
It sounds like you have two Apple ID’s. Close the one with your student account and add/change the email on your personal account. Use “forgot password” with your student email to get into your student account if you don’t know the password.

BTW, r/AppleMusic says you have 90 days to pause a subscription without losing your data:


  Your library is kept on the server for 90 days - meaning, as long as
  you restart your Apple Music 90 days of the previous subscription
  coming to an end, you'll get all your library back.