Well...

This more of a Japanese kind of place, so it would probably be delinquents instead of full blown gangs if this was operated out of a school (and delinquents do fight, but usually don't do too much stuff with drugs or weapons).

I guess though messages and services like shoplifting and other delinquent behaviors that are illegal could work (like fighting rival schools, getting cheat sheets for tests, and other things). Delinquents usually don't get involved in too much serious crime.

A full blown gang for a high schooler may be too much. Japerica also has the whole no gun thing like Japan does. So guns would be really hard to come by, next to impossible for some high school delinquents.

As for the martial arts. That I can allow.

I know what I want to give him (you get a few cool things since you gave me some nice writing and a cool CS and alot of details were put into it)

Just change the backstory up a bit to either make him an adult yakuza/gang leader or a high school delinquent group leader.

Those are much more plausible than high school gang leader.