In the Demo There are Red Walls that read Demo on them boxing you into the game area. You can leave the demo area by doing the following:

Get into a car, drive next to a demo wall making sure the drivers door is right next to it and exit the car. If you did this correctly you should be on the other side of the wall. On the other side of the wall inside the basket ball court you will find an AK-47. If you walk too far then the Demo will end.