hi
Dont worry that he stops trotting when you click. The horse will realise that it is the forward going trot that made you click so will offer more nice trots like this in an attempt to make you click more. Dont think you are rewarding him for stopping, the click is the reward not the food and as long as you dont click the "stop" the horse will not think the treat is being given for stopping but for trotting nicely.
You will be clicking a lot at first and shaping a few strides of nice trot till you have a nice trot all the way round the school eventually you will have forward going horse which needs clicking very little. Also think of the "stops" as transitions which is the best way of strengthening and improving a pace.
Hope that helps a little

Becky
