And the technical term for this is priapism! Named after some Greek fertility god who always walked around with his fat cock hanging out. I learned about this on my own when browsing Wikipedia a while after that incident, and I remember being kind of underwhelmed, like the answer to my question was so simple that my teacher really should've just known about it to begin with.

Incidentally, I think that incident was related to the laws on direct-to-consumer advertising. I believe I'd recently been in the USA at the time, and had seen a Viagra commercial on TV while there, which of course included that iconic line "call a doctor immediately if you have an erection lasting more than four hours" — meanwhile in Norway, direct-to-consumer advertising is banned, as it is in every country except the USA and Aotearoa. And this would then mean that whereas "4 hour erection = medical emergency" is a part of the sort of collective cultural awareness of the USA, thanks to these commercials; in Norway you'd presumably only really hear about priapism if you'd personally been prescribed medicine for erectile dysfunction (or otherwise were just particularly interested in the human body, or particularly tuned into the USA's pop culture).