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. :)