HELP: Captive Portal by Google authentication and ~1000 users

[sw.mok]

we're a high school in Hong Kong, China
there are about 1000 users (teachers + staff + students)
and each one has a Google account with domain of our school

Task: Only those who has our domain Google account can access the Internet
1) how can Portal do the authentication of these Google accounts?
2) how can I do the batch upload of these accounts (~1000)?