We dont normally shower after gym class at my school. We're allowed to, but nobody does. I've only heard of 2 people ever showering, and one of them had to because he fell outside and he was covered in mud.
If people normally did though, I wouldnt. I'm just not comfortable that way. I dont think I'd get a boner, but I'm a little overweight, and thats part of the reason. But if I wasnt, I still wouldnt shower.