[VLM]

Technically its not "out of" because in the linked article:

"Hunter ID number (a number which I could validate but not easily trace to a particular person)"

I'd read that as some minimal formatting and the correct number of digits and you're in.

I suspect there were no small number of school admins, teachers, parents, ex students, middle/grade school students, outright adult pedos, etc.

More like 500 registered users out of a billion internet users.

[thaumasiotes]

He also points out (just a few words from the bit you quote) that virtually everyone registered with an email address that included their name, so he knew who everyone was.