Teenagers are somewhat more difficult to handle as nagging hardly works. What I would suggest is to explain to them that during the exam period, their focus should be revision of work. Like what Yuna suggested, I would disable the wifi for a period of time. For example, wifi would be switched on after 9pm IF they have done their revisions in the day. HP usage will not be restricted, but they are responsible for any extra charges incurred. To be fair, everyone in the family will not have access to the wifi. Parents included. Just a suggestion for your consideration. :)
My dad used to switch off the internet/wifi at home and he was the only one who knew the password. This definitely limited my brothers' and my usage of the Internet. We didn't want to use the mobile data too much because we knew it would come out of our allowance if we exceeded the cap. It will be hard to control their usage outside and whether or not they will call back but you can win half the battle by limiting their usage at home by trying what my dad did. It might be a wee bit extreme but it worked haha.
As a teacher, I advice parents to set ground rules and to do up a timetable for gadget usage. Explain to them firmly yet gently on the rationale of limiting gadgets- unhealthy for their vision, reduce their focus ability, and helps them to organize their day better. You may have to even put your foot in to remove the gadget completely. Sounds rigid but with proper communication and increasing the quality time spent with your teen, this problem is often resolved.
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