Your kids are smarter than you think, especially when it comes to getting what they want. Just because you might have all these tracking devices hanging off of them doesn't mean they can't get around them. Teenagers like their privacy—they're almost adults, after all, and if you won't give it to them, chances are they'll find a way to get it. There are innumerable Internet resources that explain how to undo all these tracking measures, from hacking the phone to just passing it on to a friend while they wander off. It's hardly rocket science, so just because they aren't necessarily getting straight As doesn't mean they can't figure it out.
Some people have pointed out that if something seems forbidden, rebellious teenagers are more likely to do it, hence many teen drinking problems. If you so strictly make something against the rules by going to the extreme length of tracking the child, what's to say that they won't deliberately go against it? Going for GPS tracking isn't necessarily an effective attitude towards child rearing.