There is an interesting self defense course offered in many cities that teaches kids how to handle situations like the one mentioned above. They show the kids how to defeat a would be attacker by doing everything most attackers don't want them to do. I'm talking about screaming, dropping to the floor and using their legs etc. Both my kids have taken it and have been enrolled in Karate class for some time, though not at the present time.
A short time later, on the return leg of a long bus ride some kid decided to 'mess' with my daughter. Bad move, she literally kicked the crap out of him in the back of the bus. No amount of technology,short of a stun gun, would have helped her, so she helped herself with what she knew. They made her the 'bad guy' (because the kid was hurtin') and expelled her. Only the threat of a lawsuit pursuaded them to allow her to return to school. (pre-paid legal service rocks)
I guess what I'm getting at is that no matter how much we try to help our kids with gadgets and gizmo's, we will fail in the end if we do not teach them to take care of themselves. These devices should not be used as a solution, but rather as a backup or aid.
Kudos to you Sandman for the way you handled things......as for myself, I had to be 'restrained.'