this is wat u need to do. even though i am 13, i know a guy with hostage negotiation skills;
1. pretend that you are his friend and say that u will get others to come to you(note- when u are his "friend", ask himn if there are any more people in the killings on the bad side. and if so, than the next stuff i say to u won't work. only a one-killer job only!).
2. u go inside a (empty) classroom and say that u are going to look for a victim and u stay there.
3. look quickly in the room for something small and sharp and fit it in ur pocket.
4. say to the killer that u have the kid and he is waiting for his death inside (make sure lights are turned off, though).
5.when the killer turns on the light, stab him in the back and steal the gun from him and point it at him(don't shoot him).
6. wait for police to arrive.
if this doesn't work, don't blame me!