The joke here is that OP is the teacher. The whole time you're supposed to think OP is a child, but is in fact an adult.
I think It's a play on an older version of the joke where OP was describing their first intimate encounter with a girl at school, and then "another teacher walks in and I got fired" or something similar.
It came from the "astute" observation that large, pigmented labia resemble slices of roast beef. It's original intent was to shame women's bodies (because the more sex a woman has the looser her vagina gets and thats why some women have large labia, or so these people seem to think) and it's progressed into a general slur. Commonly used among the usual suspects; 4chan, incels, alt-right chuds, misogynists in general
If the story being greentext didn’t give it away as being fake, then the teacher sending a kid flying across a room with a single punch and not just splattering their skull where they stood definitely should have.