When the main character has to strip because he lost the game, he lowers his boxer shorts, but when he walks away from the table a few moments later, you can see he's wearing underwear again.
When the two guys first start in the Female Studies class, the teacher starts scolding the guys for being there. One of them starts pulling his knit cap down over his face, but in the next shot, the hat is back on his head.
In the opening, when the "mystery girl" is shown in parts, she undresses and is nude. But when the camera pulls back and her whole body is shown, she is clearly wearing underwear, at the very least.