Women actually used to be a larger part of I.T. in general and classrooms at one time were on track to reach an expected 45%. Instead of climbing, those numbers are falling.
You can blame it on parents not giving girls tech toys, and yes that may be part of it, but considering how many female gamers there are (nearly half of gamers are female), that doesn't explain it completely. Remember, the average age of gamers these days (30's) have kids in high school and college, that means these kids likely grew up on video games. There is obvious interest, and there was an interest, they just don't have an interest in a career in those fields any longer.
So the real question is why don't they want a career in I.T.?
The answer is men. Male geeks as it turns out are quite sexist (and geeks develop a minor god complex to boot). E3 has a spectacularly bad reputation among women and there is/was an actually boycott because of repeatedly being groped, talked down to and generally treated bad. Look at how things are marketed there... Booth babes? How much more sexist can you get. Other tech trade shows aren't any better.
In online gaming, there are a TON of women playing games online, you guys act like they don't play FPS or anything hardcore, yes, we do, you just don't see them because most of them hide their gender. Xbox players are the worst, however even on PC, which is better it can still be pretty bad. Personally, I don't hide my gender, however I won't play before 8pm, and I won't play long unless it's with other friends. All it takes is one guys to ask "are you really a girl", and the entire game will change and go downhill. If it's not "you got killed by a girl", it's a guy trying to talk smack to me. If I stay quiet, he will get more aggressive as it goes, if I talk back, he will instantly go ballistic, there is no middle ground, it usually only stops if a 3rd person steps in, I leave or someone gets booted. Then your have the games themselves, IF there is a female character, her clothes get skimpier the more "armor" she gets... By the time she is "armored up" she looks more like a sex slave than a warrior. Which only reinforces the males view of women only being good for eye candy. Try playing as a girl for a week or more and you will begin to see what women really go through when playing online and you will begin to see why they hide.
This is why the classrooms are emptying out. I know not all guys are this way, and as they grow up they grow out of this (some more than others), however the worst of it happens right at the age where we are choosing our careers (teens). Why would a woman pick a career where she's not only objectified, she's verbally abused on a daily basis?