If you already have a Soundtrap account and want to join an Edu account, you have to get either a code or a link from your teacher (or the owner of the account).
Once you got either one go through these steps if you have a CODE.
(Teacher/Owner finds code through Edu Admin panel - Import Users - Copy code)
JOIN BY CODE
1. Log in to your Soundtrap account.
2. On your profile page click on EDUCATION
3. Click on JOIN WITH A CODE
4. Type in the CODE in the field and click OK
5. Confirm to join the Edu account. Click I CONFIRM
6. Welcome to your group/class!