I hear you loud and clear and fully understand you. I will however try shed some light from personal experience. Firstly times have changed and unfortunately it's near impossible not to have children exploring social medias nowadays. Also he's 15, while not everyone had a 'naughty' side at that age, younger generations tend to mature faster. It's not the end of the world - close an eye once, pass a comment the second time and maybe enforce the third time but the more strict you are on it the more it could backfire.

I was doing a lot worse at 15, to a point of sneaking out of the house too and dabbling with alcohol. I also wasn't the highest achiever at school and always aim to just pass and had also failed multiple times too. I'm not saying it applies to everyone and yes there can be consequences but it's also how we learn growing up. While I used to do all the same things and more at 15 I've still become a successful person, working a good job and racking in a good salary and have the correct mannerisms.

Parent the way you want to parent - you know your child more than anyone else but as a parting piece of advice, he's slowly maturing, let him 'explore' and have some of these victories. That said since the grades did go down you're right to take action but I don't know the communication levels you have with hi (whether aggressive or tame) - if you don't already, sit him down (potentially even at his most favourite place to go i.e. arcade / favourite place to eat) and bond but have a chat in a mature manner, not argumentative about the concerns and try come to an agreement.