perhaps a grep of the firefox file logs for all the students is in order ?
grep google.com /home/USERNAME/.mozilla/firefox/*.default/history.dat
With a bit of tweaking you could have a bash script that would let you search the logs of all the students ..... and enter a specific SEARCH term.....
#A bash script to search firefox history logs on all users....
read -p "Enter a website or url to search for : " searchurl
grep -H $searchurl /home/*/.mozilla/firefox/*.default/history.dat
# Should print out a nice list and get you the info you need as long as you submit the url you want to search for ;)
ends up looking something like this when you run it......
crouse@linux:~/Scripts> sh firefoxhistorysearch.sh
Enter a website or url to search for : midiowacamaroclub.com
/home/crouse/.mozilla/firefox/wb5xfjqk.default/history.dat: =midiowacamaroclub.com)(F2F6=M$00i$00d$00 $00I$00o$00w$00a$00 $00C$00a$00m\
obviously the offending party will have his/her username showing up right beside the offending url