Our statistics program performs DNS lookups based on the log files, and returns us "most frequent visitor" statistics. Most K-12 schools have "k12" in their name.
It also provides us with a very nice "last 300 visitors" page, which is constantly updated.
They're probably finding us through Google, as that is our most popular engine. People have also found us in these other search engines this month:
Ask Jeeves
AOL
Yahoo
Dogpile
Metacrawler
Alta Vista
Dmoz
Netscape
Excite
Mamma
Alexa
Mike (I'm not a search engine

)