Pandora the sex magnet just sounds like a .... i'm not even going to start, she is immature though.
It's perfectly normal. you are 13 after all. during that age your hormones are very active any even the same gender can make you have an erection. Well at your are you get erections a lot..trust me i remember...
try to think of something unpleasing like the person said of "your grandma naked" that sounds like it would work, or just look away.
sorry to say, it's gonna happen a lot