If there are only 5 boys and 5 girls in each house in each year, then i assume the over all number of students is around 280. Thats only based on the fact that there are 5 boys in Griffindor, in Harrys year(Ron, Neville, Seamus, Dean and Harry). you're wrong :)
There are five boys/girls in each dorm room. Rowling said that there were 1,000 in the entire school. There were hundreds anyway.