Our lives would be simplified mightily if our 12 yr old could bike 3 miles home from school by herself sometimes. If she cuts across the park she avoids the only scary bit. The rest is bike lanes and quiet roads. Here's what I've done or will do to prepare her:

She's familiar with the route to school, but I'll ride home with her a few times so she's familiar with the route in the opposite direction.

I've gotten extra CO2 cartridge, spare tube, tire levers in her seat pack. I'll teach her how to change the tube, using up a CO2 cartridge so she knows how to use it.

I'll show her how to put back on a slipped chain.

I'll get ID in her seat pack, with my contact info.

I'll tell her if anything besides a flat or slipped chain happens, walk the bike home if she can, lock it up and walk herself home, or just leave it off the side of the road and walk herself home. It would be a long walk but this is next-to-worst-case scenario and probably won't happen.

I'll recommend she wear my reflective orange road vest but I won't insist. There are some things that are too much to ask. I know she'll wear her helmet.

I don't want to give her a cell phone, and I don't want to be a fear-mongerer about strangers. She's not going to be murdered or kidnapped.

Any other thoughts?