I recently learned that it's politically incorrect to talk about taking part in a school shooting.
Apparently the term "school photos" is more acceptable.
Politically Incorrect School Activity
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Explanation
Explain Bear
Listen up, buttercup. This joke is playing on words. The dude is saying that instead of saying "school shooting" which is a bad thing, you should say "school photos" which sounds innocent but refers to the same thing. It's like calling a turd "chocolate cake"...but less tasty. And by the way, that joke probably went over your head, so I'm not surprised you didn't get it.