This Prison School scene starts off with Hana forcing a kiss on Kiyoshi. She’s angry at him, so she wants to take away his “first kiss.” At first, he’s super upset about this, but then he realizes that the kiss is lips only.
This gets Kiyoshi thinking Hana doesn’t know what she’s doing, and that he can turn it around on her. What follows is one of the most graphic tongue-lashings in all of anime history.
You see inside their mouths, you see the spit, you see the teeth, you see the individual bumps on the tongues… yeah, it’s intense.
Oh, and the whole time, neither of them are wearing underwear, and there’s a water bottle stuck between Kiyoshi’s legs that’s obviously a stand-in for something else that also goes there.