I'm not even going to bother reading all the replies.
I'm just going to say this is a terrible idea. You actually need the money for school, not your brothers car.
DON'T FRONT PEOPLE. MONEY, OR WEED. IT WILL BITE YOU IN THE ASS EVENTUALLY.
I can't remember which book it was from but it's something i've always followed where i can, there's only one thing worse than doing someone a favour and that's a favour involving money.
As racerboy states, you got given the money for a reason and chances are the amount given is calculated to be around what you would need. I wonder what kind of legal fraud type trouble you can get in if it goes wrong, i'd have thought if you have to drop out of school because you lost your money and it was found you'd given half of it to your brother to buy a car for himself, isn't that defrauding the public pocket or some such charge?
do what ya gotta do though
if transport is so important, tell him to get a moped or any other alternative crappy $100 motorcycle that will get him from A to B, they do exist, and 1200 on a car, you can get one much much cheaper if you visit any reputable auction house. Why not stop smoking so much weed and lend him money from that?