Student account creation - Google Apps for Ed email accounts - closed to outside district

Hello. Our students each have a Google Apps for Edu account, and the students GMail addresses end with the district domain (@s.dcsdk12.org). In our school, we've chosen to keep those email accounts closed to any other email addresses outside of our district. Students can email each other and receive emails from teachers, but the accounts block email from outside the district. This means that our students cannot receive the confirmation email from Prezi.

We do not want to set up an additional email account (through Gaggle or similar) just to create our Prezi accounts.

Is there any other way that our student accounts can be confirmed?

Thanks!