Because Osaka killed the crap out of pretty much everyone else (Shot Yomi and Sakaki, though Tomo didn't know about Yomi getting killed, stabbed Kagura through the MOUTH, and Chiyo through in the throat. Real graphic stuff, but at least the manga ends on a happy note.
The hentai in Azumanga Drifting Classroom is so tacked on it's not even funny. Seriously, it's literally stuff Tomo thinks up and subsequently gets whapped for.
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