Hi M., We had the same problem with my daughter and we ended up getting an alarm system not because of her, but for other reasons. However, when we got it, I had them put alarms on all the doors so that I hear a beep if a door or window opens. It can be annoying because it isn't easy to turn off, so we never have, and when the kids are trying to fall asleep, they are alerted to when we come and cry if they think that I am leaving.
Anyway, some things to look out for, Broadband is inexpensive because you pay rent on the installed equipment and end up with a higher monthly fee. If you pay a lot for the installation and equipment make sure that you own the equipment in the house, and ask what the monitoring fees are. We used Viking alarms, and we own the equipment, and pay less then 10 a month for the monitoring service (you know, they call you when an alarm goes off, then you give a password, and if it isn't the right one, they send the cops). The phone number is ###-###-####. It is a really small operation. Again the equipment/installation cost a ton, but when we sell the house, there won't be any issues with who owns it and you can switch monitoring companies if the new owners want to.
I use it as an alert to tell me when my daughter is disobeying the rules, and I agree with the other mom that they need taught the rules but while in the teaching mode, it is better safe than sorry. Good luck.