Why make him a High School student?
Convienience. They can set up a half dozen things in about 15 minutes with the school. Where as starting in the mountains would take about 40 minutes. I worked up a quick plot time-line thing just for kicks earlier, to see how the start of the movie could flow. The school situations opens up many options for establishing important things for the story. The eclipse we've heard about, Goku's strength, Goku's naive and rather clueless nature (seriously, think about it. How much more 'fish out of water' can you get for Goku than dropping him into a highschool? (while still keeping it in the realm of believability, hence not being in a college, he'd never get that far (I'm not even sure he'd manage to graduate highschool XD (Man, I've really stretched out this sentence with the parentheses)))) and so on.
Sure, this material could be established in the mountains as well, but it would take a little longer. First you'd need to establish Goku's solitude up there, then introduce Bulma and hint at his strenght, establish Goku's naviety and cluelessness, fully display Goku's strength, and then get them on their way to a more populated area while somehow working in information on the eclipse. Then, once they're around a larger crowd you could expand on Goku's nature and show he's not just somewhat out of touch with the world but completely.
Really, in the end, it's nothing more than a simple time saving convention that will almost certainly be otherwise ignored and forgotten, never to be mentioned again in the movie or the sequels.