oh chow yun fat, how you have fallen. i remember when i used to think of you as a total badass.
Actually, HK action cinema features a lot of slapstick or character exaggeration that mixes in with the slick action. Chow isn't immune to this; he played a gambler who became a comic-relief retarded man for instance.
So, I agree if this movie was made in the vein of Shaolin Soccer and Kung Fu Hustle, it would've been a good movie, or at least more faithful to the DB manga. This way, Chow can be as perverted as his chara is supposed to be, and it'd be okay. You can have both Mr.Bean-type gags and Hiroshima-leveling explosions in the same HK movie... Americans aren't quite used to that.
Goku should not be a high schooler. He never went to school. He's not the type of person to go to school anyways. It's true that he is like a wild wolf-child. And this story needed to take place in the alternate-Earth the manga takes place in, with its egg-like buildings and strange anthro-citizens, in order for the society the story takes place in to even make sense.
This movie is like taking LOTR and putting it in Europe circa 1000AD, and then expecting the movie to make sense.