Under their school robes, the students all seem to be wearing school-colored ties and gray sweaters with school-color detailing. This would be a really easy sweater to make, it is probably all stocking stitch and drop shouldered. And actually, this would probably be a great sweater to buy store bought, and then add the color details. All you have to do is buy a gray v-neck sweater at the store, then duplicate stitch scarlet and gold stripes around the neckline! (and maybe around the wrists, too. Though not at the very edge of the wrists, as we can see them and they're gray) Talk about a last-minute gift. This might also be a good thing to make if you don't want to be screaming "Harry Potter Fan!" -- only true fans will even notice you're wearing something HP related.
Update 11/28/01: After seeing the movie, I noticed that ALL the students wear this uniform. Each house has its colors on the edge of the sweaters and on the ties. The best part of this is that when the new students arrive, they are wearing PLAIN gray sweaters and plain ties! That was a great detail that they added to the movie. When the students enter the hall to be sorted, their sweaters are still plain, and after they are sorted the sweaters stay plain, but the next day the colors are there! Pretty cool. It's not clear whether they change magically (which wouldn't fit too well with other things in the book, such as Ron's ugly dress robes in book 4), or if they're given new sweaters and ties along with their scarves and other school-color stuff that they receive that first night.