If last month's epic bus driver fight taught you nothing, let this new video serve as a final lesson -- DO NOT MESS WITH YOUR BUS DRIVER ... because she will beat the crap out of you.

The new video was shot yesterday on board a Baltimore bus -- showing a female bus operator unleashing a ferocious beatdown on a female high school student.

According to the person who posted the clip on YouTube, the fight started because the student was playing her music too loud -- and the bus driver hit FIRST.

At the end of the video, the bus driver says the fight was about the student being "disrespectful."

A rep for the MTA in Baltimore says the driver has worked for the city since 2003, and has been placed on leave while the MTA investigates the incident