Admins can now renew school accounts directly from their account screen.
First, ensure your teacher list is updated by clicking on the Go To menu and selecting "Teachers". Here, add/remove teachers as needed.
After that, click on the display name in the upper-right corner of your account and select "Account". Here, you should see an option for "Renew Subscription". This will allow you to choose the number of teachers you wish to renew and how far into the future you wish to renew for.
On this same page, you can also choose how to receive your invoice. You can also pay the invoice on this page with a credit card.