iPhone 4S randomly turns on

Whenever I turn my iPhone 4S off it will turn itself on again, it usually happens about an hour after I turn it off but it varies. It's been happening for about a year now and it didn't really bother me because I just left to on charge and I rarely turn it off anyway. However, I have my A Level exams at the moment where my phone, obviously, has to be turned off. I've had to leave it at home because of this, however for one exam I have to take my phone as I have plans after the exam. Does anyone know how I can stop this happening? :confused:

-I have the latest update
-The power button isn't (obviously) getting stuck
-It isn't jailbroken, and I don't have any apps that could have given me a virus. (Well I suppose I could have, but they're all really common ones like Twitter)

Thanks for any help! :)