If you can't migrate your account because the admin of the main account left the school or you don't have access to that email anymore, we can manually change the email address with a new email address.
To be able to do that for your school before the migration of the data, we would need an email from the headteacher or a senior leadership team member to request this change at firstname.lastname@example.org. The email sent by the headteacher/SLT should include their signature, and clearly state the previous email address that is going to be replaced with a new email.
If you can access emails sent to the main account, we encourage you to migrate the account as it is and only after your migration is successful make the relevant changes to the account on the new website. Please see how here