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.
Surprisingly high percentage, but I guess in such a relatively large group the negative effect of someone from that group having access to everything isn't as large as it would be in a smaller one. I don't think that would have working in my highschool, at least not without encryption and source code available...