((((((((((hugs))))))))))
By all means talk to the school resource officer. Here that person is a cop stationed at the school, is it the same where you are? I would also speak to the gang unit of the police in your area. Gangs are nasty and highly dangerous. It is VERY likely that your son is using drugs - WAY more than pot. This is all part of gang life and is almost expected.
I don't know what you can do to keep him in the house. At his age you truly cannot do a whole lot to him. What you CAN do is to stop giving him ANY money for ANYTHING. Period. If he needs money for lunch at school, go to the school and pay the money to them directly. If he is doing some sort of sport or activity that he needs money for, ONLY pay if you can go give it to the adults in charge and verify that he is truly doing this.
You are required to give him a roof over his head, food in his belly, and clothes to wear. NOT the clothes he wants, the food he wants, or a nice bedroom. Strip him of EVERYTHING except 7 outfits of YOUR choosing (avoiding his gang colors, etc... thrift store clothes are just fine for him if he needs anything new - including shoes), a mattress on the floor, one sheet, one blanket and one pillow. Food that he likes is to disappear from your home or be locked up in something he cannot get into. He will need a light of some sort in the room - if there is a ceiling light that is fine otherwise a cheap lamp.
You are FREE to get rid of his stereo, music, ipod, posters, books, and EVERYTHING else. He does NOT own ANYTHING - you are his mother and have a legal right to remove anything you want from his possession and your home. You cannot lock him into the home but you CAN lock any room you do not want him in. I would put a sturdy door on the bedroom with a deadbolt, and put locks on every other door in the home. Keep any $$ you have either in your pocket or in a safe in your room. One parent here had her teen use a knife to cut the pockets of the pants to get the little money she had out of them so he could go party. He cut the pants while she was wearing them to sleep in!!
If your son touches you in anger, call the police and press assault and domestic violence charges on him. EVERY SINGLE TIME. If he sneaks out of the house, report him as a runaway. If he comes home stoned, call the cops. If you find alcohol or drugs in the house, report it to the police. It will take time before they do much, but keep making reports. It is really the only consequence besides taking all of his stuff away that you have.
As for his stuff, let him EARN it back with good behavior. If he brings other stuff into the house, get rid of it. If it is something expensive that he might have stolen, report it to the cops as possible stolen property and let them take care of it.
Your son is going to be furious, but I bet you already are furious at him. Make sure he knows that if he so much as touches you in anger, or if he threatens you, you WILL call the police and you WILL press charges. If he becomes a danger to you, you can tell the police that he cannot stay at your home. They won't like it and will try to talk you out of it, but they cannot refuse to take him if you won't let him back in the house because he is a danger to you.
Sadly, by age 15 if a difficult child is determined to be in a gang, or hang around the wrong people, or use drugs, or commit crimes, we parents have very few ways to get them to change. Unless and until difficult child wants to change you situation is going to get a LOT worse before it gets better. If you have younger children be very careful to make sure they are not home alone with him. Kids in gangs often use younger siblings to do various things so that they won't get into trouble. They also frequently hide weapons/drugs/contraband in siblings' rooms so that they won't be caught with them. Make sure any younger kids have key locks on their doors also, so that they can be safe from difficult child if he decides to hurt them in any way.
I am so very sorry. I wish I could say something more hopeful. I hope the police are able to give you some ways to get him out of the gang if that is possible, and that they will help you with him.