Ultimate Frisbee

Ultimate Frisbee is a game we're playing at recess at school right now. It's pretty fun if most of the players are good at throwing frisbees. Otherwise, it's a waste of time.
How to play:

1. Find a playing area
2. Near the two ends of the area, use two markers (sweaters, rocks, etc.) to mark an invisible line which will be the goal (the frisbee can go anywhere as long as it passes over the invisible line)
3. Divide into equal teams (well doesn't really have to be equal but that would be fairest in most cases)
4. Decide which team has to start
5. That team starts with one of the team members with the frisbee in the center of the playing area and tries to pass to other players on his/her team.
6. Once you are in possession of the frisbee, you cannot take any steps.
7. When someone on your team catches the frisbee (without dropping it, of course) behind the invisible line your team wins a point

If someone drops it, it's the other team's frisbee
If someone from the other intercepts the frisbee, it's theirs
Whoever touched the frisbee last before it dropped, the other team gets the frisbee.

Tell me if the instructions are unclear or incorrect. This is how we play it, do you have any variations?