Dragon Quest: Battle Pencils!
Much like Pokemon is short for Pocket Mosters, Batoen is short for Battle Pencils… just the word for pencil in Japanese is enpitsu. It’s a simple idea. Take pencils, write stuff on each of the sides, so that the pencil becomes a sort of six-sided die. You roll the pencil and the pencil, which represents a monster from Dragon Quest, does something, like damage or healing of hp, or other interesting effects (One kid had some hot chick monster, it’s effect was, “Monster kisses everyone, they are made happy”, more detailed rules after the cut).
The kicker is that you are playing with pencils. Japanese schools are pretty strict when it comes to games. No cards(except for school sanctioned “Japanese cultural cards” like karuta or hyakunin …), so poker and Magic/Pokemon are out. But the fact that this uses pencils means that it flies under the radar most the time. And even if the teachers did know about it, what are they gonna do, take the kids pencils away?