This sounds like a simple exercise but you will be amazed how few manage to execute the exercise perfectly first time!
Use two poles, set with two or three canter strides in between (depending on your school's dimensions) across a diagonal of your school. The exercise is to canter across the diagonal, over the poles, with even canter strides, and then come to a halt on the diagonal line at the school end. If you can't stop, go around the school's short side and halt as soon as you can. Try to halt sooner on your next attempt.
Do this exercise from both reins - you may well find it much easier on one rein.
The purpose of the exercise is to get your horse listening to you, ensure you have control and can influence your horse. Riding specific lines also helps you practise getting the correct line eg for riding a dog leg approach to a fence.
For more jumping tips click here