Let's review the school administration's argument:
"If this kid is allowed to wear canerows, some other boy might join a gang."
That's not even a failed syllogism. It is not a reasonable sounding argument that runs afoul of the informal fallacies.
The argument is absurd on the face of it.
Now sure, a school can make a sensible policy such as banning gang tattoos, jewelry and haircuts. (Symbols cut into the backs of heads of hair.) That system was standard for Los Angeles Basin schools last I was there.
Canerows ??? That's not smart.