There isn’t much you can do about a sucker punch.
But man, the bus is probably one of the worst places to get into a fight. Confined af, unpredictable environment, and a bunch of people who are all X Factors.
But you gotta protect what’s important to you. Maybe next time just move to another seat. Some people will engage with you just to fuck with you. Best solution is to just not be there.
If the fight is inevitable, don’t get caught sitting. That’s just combat tactics 101. I’m a big fan of making the fight unfair. Not sure where you live, but if you can carry anything as a legal weapon, I’m all for it.
Other than that, martial arts training probably wasn’t going to help this situation. You got suckered. What you needed was familiarity of conflict and some street smarts to identify and avoid the threat.