This is a confusing part of HP, and I've read up on it a bit before. What I think is that there are 5 boys and 5 girls in each house for each year. So there are 5 male first year Gryffindors and 5 male second year Gryffindors and 5 female first year Gryffindors. this would equal out to 280 students at Hogwarts. Which sounds right in some places and doesn't in others. So there are the same number of boys and girls. Make sense? And maybe I am wrong.