If "pork and beans" can be singular (10th grade english trick question!!), than so can martial arts, but it sounds weird either way, so I'd stick with plural!
Not the kind of school I'd like to be in. You could also move to China and join the state-sponsored shaolin temple tourist attraction as a martial monk.
i think whatever feels most comfortable to you. well i think you should do boxing because why did you quite martial arts in the first place?