You should definitely press charges.
My son also just started middle school, and it's definitely not the safe haven that his elementary school was for the past 6 years. He's gotten a couple light shoves in passing from some other kids. He said it's not personal, just older kids who want to throw their weight around, and it's not enough to dampen his enthusiasm for middle school. My daughter, who's now a junior, said that happened to her on occasion when she was a sixth grader. Let's face it -- middle school sucks, and you're going to find a lot more assholes there than in elementary school. I've always tried to avoid being a helicopter parent, and I think it's good to just let your kids navigate these experiences on their own without going to a teacher or principal. They'll be better for it.
That said, when it comes to physical harm to my kid, or something that rises to the level of targeted bullying (thankfully neither has ever happened to my kids), that's where I draw the line and throw off the gloves. I know it's a 7th grader and I'm guessing there's a chance that her home life might be less than ideal. But fuck her, especially since this isn't her first transgression. Your kid could've been hurt a lot worse. She needs and deserves the most severe consequences available.
I'm sorry this happened to your son, and glad it's not worse.