To continue with Premium plan after trial, you can simply let the subscription continue after the 7th day (whereby you will be billed with the annual subscription fee and your account will be converted to Premium).
Thereafter, you can fill up this form to request for the merger of accounts for seamless access. You will be able to toggle between the Premium & School interface with the final merged account.
If you have any further enquiry, you can reach out to us at support@koobits.com.