This is one of those things that tends to work like this: If there is no problem, you're okay ... if there IS a problem (illness, fire, police response, or other emergency) you CAN be in trouble.
As the father of three sons - two of which are 12 and 14 - I would never consider leaving them un-supervised three days a week from 5 PM to 2 AM! Too much time to get in to trouble and mischief, too likely that friends will come over and problems almost ALWAYS come from that, much too likely that homework will be neglected, etc.
If this is a three day deal, it's a judgment call. If it is every week for an indeterminate time frame, then I would think it to be a foolish act to leave them home alone. Even the best of kids get into trouble if given too great a chance ... and, at age 13, boys tend to lose their brains and common sense for a few years.
- Carl