Getting expelled from a school is a step beyond suspension. It means that you are asked to leave and never come back. In other words, you have been kicked out.
More benignly, expel can mean discharge: if you're having a water-fountain spitting-fight, you are hoping to expel water from your mouth in the direction of your opponent before they can get you. But don't do this; you won't get expelled from school, but the germs in your body will be expelled all over your friends.