Turn off your wi-fi, take her ipod & cell & when she does earn them back, give her time limits. My oldest learned how to "pirate" the neighbor's wi-fi, so even when I turned it off, she was staying up half the night on her laptop. She started failing classes, due to lack of sleep & because instead of doing her homework, she was spending her time on her tumblr, facebook, etc. We took her laptop eventually, because although we were giving her reasonable time limits, she was sneaking & using her laptop. She can use her laptop ONLY at school, for a school project. I cancelled the data plan on her cell, so it is just a phone & she doesn't text much, but if she were to start, we'd take that, too.
All of our girls know that they have to turn off the screens--cell, ipods, tv, laptop 1 hr before bedtime. If their grades go down, they have a bad attitude regarding chores or a bad attitude about getting off of their devices, they lose them for the week. Yes, they need the internet for homework. I either supervise their internet use or they can go to the library & do their homework there.
Screens are addictive. The sneaking...not good at all. She's only 12, it WILL get worse, if you don't crack down & establish healthy internet habits with her. Good Luck