Just looked at a school calendar and saw that your teens are heading into the dreaded mid-year exams and grade time. At least when this all happens in June, the kids know that on the other side of it all is freedom!!! Now all they have to look forward to is five more months of school. Tough to find the motivation. I feel depressed just thinking about it and I don't need to study for anything.
If you have a Freshman, this is their first go-round with the whole high school exam thing. It can feel daunting and terrifying. Perhaps they have dug a bit of a whole for themselves, and have no idea how to get out. They need your help to develop a strategy. Just telling them to go and study is not helpful. They may have no idea what that means. How do you review 5 months of work in 5 subjects??? Help them come up with a game plan. Maybe take them to the public library to study so they can get out of the house and away from some of the temptations that are there and that will distract them when they are feeling overwhelmed. Use this I get it moment: " I get this whole exam thing can be very overwhelming. Lets go to the library for a couple of hours. Pick one subject to focus on, and then we can get some coffee/ice cream/pizza on the way home. Staying in the house will be too distracting. This way when all your friends are looking to distract you by texting and posting on facebook, you won't be tempted to response, at least for a few hours." (hold on to their phone for them when you get to the library) If your teen prefers to stay home, make sure you schedule some phone free time. You can tell them that it isn't a choice whether they give it to you for an hour, but they can choose which hour to give it to you.
By the way, having them give you their phone, and staying off the computer for an hour or more a day should be for all your middle and high school kids except for the seniors. If your seniors haven't figured out yet how to study, there is not much more you can do. It is totally on them to figure it out. You won't be around next year to hold their hand and bring them snacks.
So back to the phone and computer thing. You need to be clear with your kids that this is important. You get how hard it will be not to be in constant contact with your friends, but it is your job to help them develop good study habits, especially when it comes to exam times. If they choose not to willingly give you their phone, I would not get into a power struggle with them but I would give them this potential consequence. " If you choose not to give me your phone and/or computer for some good study time, and if your exam and mid-year grades are not your best effort, (meaning their grades suck)the consequence will be that will switch out your smartphone for a normal phone, (or for those old fashioned kids who still have a flip phone) we will start the new term by turning your phone off for study hours, so you can improve. If we see you putting a good effort in now,(giving me your phone and laptop) we won't need to do that.
As I have said many times, my college students have told me time and time again they wished their parents had helped them monitor their phone and facebook time during high school, because as college students they are completely out of control. You don't have to yell, you don't have to scream, you don't have to criticize, you don't have to judge, but you do have to parent.