Is it normal to want to masturbate if I'm around a lot of hot girls in gym class?

[phillip]

I'm 15 and everytime I'm in gym class at school and I see a lot of the hot girls I really get this urge to masturbate. I also tend to get an erection a lot in gym class. I can't help it, its just seeing them all in tight little gym shorts, it really gets me worked up a lot. It also doesn't help the fact that my girlfriend is in the same gym class as me. I know I can't masturbate in public obviously or at school but it just really gets to me because I get so worked up and turned on by them. I have to wait til I get home to do it. Plus the fact I wear boxers under my gym clothes doesn't always help sometimes if I get hard. Even just thinking about it gets me a little hard down there. I don't want my girlfriend and the other girls to think I'm just after sex, but I can't help it, its really hard not to think this way and feel like that. Is this normal at my age?