IMO, You probably did the right thing as far as your career and livelihood goes.
However, I can't help but feel that if this punk is 19, then he is, by every standard we have, a legal adult...student or not. And a legal adult who assaults someone is subject to adult consequences...and you should have dropped him cold. Then slap him awake, and deck him again.
Clearly, it takes a real man to push a guy down a flight of stairs, probably from behind, then run like a girl (please pardon me, ladies...just an expression) to avoid facing up. Little budding monsters like this need to see the other side of adult freedom...it's called consequences.
On another note...Crips and Bloods in
Memphis ?

Wow. I'm not sure I ever imagined such a thing. Maybe this shining light should take a good look at where 'Tookie' Williams is now...
Remember, man...if no one saw you punch his punk lights out, then he 'fell'.
I sincerely hope you stay safe, and that your friend recovers well and quickly. Hell of a Christmas present for people who work to give kids a future.
In a perfect world, you and your bud will wait till the noise blows over, find the criminal's route home, and corner him alone. You know...use his own MO. Explain carefully a few facts of life, and then beat the living shiatsu of of him.
Actually, this is more noise than good sense. I just get outraged by stuff like this, and ungrateful little crapheads who fancy themselves 'urban warriors'. Pffft. My ass.
, and be well.