It sounds like he has more freedom than he needs, and he's moving in the wrong direction with that freedom.
The phone is just a tool.
Do you want him to be able to contact you when he's not at home? An old push-button cell phone will work fine. He doesn't need a smartphone, he can call or text you from an inexpensive prepaid phone. Since he has the phone just to communicate with you, you shouldn't need to load the phone with more than $15 or $20 a month. If he wants to call or text his friends too, allow him to earn minutes or money to add minutes to his (your) account. There are phones you can purchase that are able to call only a handful of preset numbers in addition to making emergency calls.
The phone is also a toy.
Is his phone a "toy" (for him to enjoy) as well as a "tool" (to keep you connected)? You should have enough knowledge about the devices you're allowing him to use that you can control his access to different functions of the device. If the device doesn't have parental controls or password protected functions (ex, for downloading new apps), it may not be the right device for him at this time. When he becomes more familiar with the device than you, it's possible that he'll learn how to cover his tracks and continue the behavior without your knowledge.
When I was his age, my sister and I began to download music from the internet illegally using peer-to-peer file sharing programs. My dad wasn't as familiar with computers as we were, but he knew what we were doing was illegal, and it was exposing both our home computer and our eyes and ears to unnecessary risk. He forbid us to download music and made us uninstall the program. When I reinstalled the program later, my dad saw it and I was punished, so this was my 12 year old solution: when I wanted to download music, I would install the program, do what I wanted, then uninstall it when I was finished. Dad was none the wiser. And He never caught me- I kept pirating music until well into my twenties, when I got a letter in the mail from a copyright holder of a file I downloaded threatening a lawsuit!
If the phone is a toy, maybe now is the time to transition to using it as a tool alone, because of his grades. You want him to be able to contact you if he needs to. Prepaid phones are so inexpensive that you may find it acceptable to replace his smartphone if he's acting up or until he brings his grades up, and just store it somewhere when he has access to the phone he's using now. Right now, his priority is passing his classes (actually, it should be learning, but I digress). The fact that he's so interested in using the phone to talk to girls could work in your favor as a reinforcement for bringing his grades up.
As for kiking with this girl, this could be the start of very dangerous behaviour. Of course, it's good that he's interested in romance and everything that comes with it- he's at that age. It's important though that his relationships are healthy, constructive, respectful, and most importantly for now, observed by you. Remember, it's your job to teach him about his desires and how to express them safely and effectively. You should be talking to him about his feelings for girls in general, which girls he likes, etc. You should continue to focus your discussion on his relationships with girls (Is she nice? or What do you talk about?), and not just his physical attraction to them (Is she pretty? What does she look like?).
He's a boy, he'll chase what he wants, especially if she's communicating that she wants to be chased. I think at this age it's not beneficial for him to be reaching out in a sexual (or pre-sexual) way to a girl he doesn't know or doesn't see in person. To me that seems like the first step on a path toward more destructive (and distracting) behaviors. Also, depending on where you live, sending erotic messages to a minor may be illegal, even if sent by another minor. Sending erotic images most certainly is. If the phone is in your name (I can't imagine it's in his!), his actions could have serious negative consequences not just for him but for you and your whole family. Talk to your son about decency and indecency, especially among minors, and especially in regards to digital media. Maybe start the conversation by talking about the difference in age ratings for movies he likes.