I finally got around to reading this thread. Looks like girls need to be better represented here!
About how to tell when a girl is on her period. Going to the bathroom with her purse is a good clue, but not necessarily the case all the time. A lot of girls carry loads of makeup with them and feel the need to touch up a lot. However if, like the other guy said, she gets up in the middle of class to do this and then comes back, it's very likely the case. My sweetheart can tell when I'm getting close to starting my period because I unwittingly start being more defensive, argumentative, and easier to anger. I often don't realize it unless it's pointed out to me, or I think about my actions later and realize that it was not very rational.
And yes, on the last day of my period and a few days after that I am extra horny. It's funny, because I'll go to my sweetheart and say, "Guess what.... my period is gone...."
