What would you do? (School grades)

Take the phone away until his grades are up.

So he managed to go from grades good enough to earn a phone to failing in 3 weeks? Wow! How did that happen? Was he not doing and turning in homework? Was he misbehaving and slacking off in class? Find out the reason behind it, if you don't already know.

Personally, my first form of discipline on this would be to take the phone away. I would take it over the break, and possibly until the next report card or progress report comes home with acceptable grades. If not that long, I'd be getting in touch with the teacher and making it clear to son that until teacher says those grades are improving dramatically, that phone is staying with me. I'd also probably severely restrict his activities over the break, lots of time at home - and if he's not doing the work and you can get the assignments, I'd be making him do them over the break. Even if he doesn't get credit, they should be done.

so the new semester started? Usually- that isn't until after the holiday...
If it is a new one, I would have a talk with him an find out what is going on and let him know if those grade don't come up- he will lose privileges and/or devices.
give him a couple of weeks when they go back to get them going back up

We do full on electronics grounding - no computer, no tv, no video games. He has to show us his backpack when he gets home, and we go through it with him looking for any unfinished work. His school does day planners for all students - they write down what they're doing in every subject, teacher adds her notes as needed, we have to initial it every night - that's the biggest help, he knows if he tries to get out of it, the teacher will add a note for us directly to the book. Not bringing his book home is instant 1 day grounding as well (or as long as it takes for him to bring it home, forgetting it on a Friday is a bad thing).

In our house if they didn't have time for schoolwork and studying, they didn't have time for cell phones, computers, video games or television. That was the rule, period. All of those things are privileges. If you don't take care of your responsibilities, you don't get privileges. It's that simple.

And yes, he would have to prove his grades were up to earn them back, and if they fell again, he'd lose them again.

I'm sorry I'm just not buying it that a kid can go from good grades to failing that fast. Either he lied to you and changed his report card ( changing an f to a b is real easy) or his grades were never that good to begin with. In either case take the phone away and make sure he does his homework and study.