[dmm]

A credit card number would be hard to fake.

A scanned pic of govt id that matches a name would work too.

[EvanKelly]

If high schoolers have access to scannable fake IDs, then I'm not sure a scanned pic of an ID is really suitably difficult to fake.

I agree that CC numbers is probably a good way forward as long as your users trust your site enough to provide that information.

Are CC numbers easier to "name check" than SSNs?

[joshschreuder]

High schoolers also have a larger net gain to faking their ID (years more drinking and partying), as opposed to winning a competition online.

But I guess it depends on the competition (eg. a digital camera vs. a car would provide different levels of motivation to cheat)