Do you wear white breeches in a jumper medal class?
You can, but they are certainly not required. The appropriateness will depend on the height and level of the class (white breeches would likely look out of place in a jumper medal class at a schooling show, whereas several riders might wear them on the 'A' circuit, especially for a big medal that qualifies for a final).
How do you know you are excused from the jumper ring?
The bell or whistle will sound repeatedly (i.e., a “beep-beep” instead of a “beep”) to indicate that you have been excused. The time, if it is displayed, should also be stopped.
How do you put on a collar?
Most chokers have a small slit on the back halfway along their length. This slit should be aligned with the lower button on the collar of your shirt. Insert the lower button into the slit (some shirts make this easier than others); this keeps the choker in place. Button up the ends of the choker at the back of your neck, make sure that everything is lined up correctly and you're done!
Is there a bell before starting a class in the hunter ring?
No, there is not. Over fences, you are to be judged from the moment you enter the competition ring until you leave it, so a bell is not necessary. In an under saddle class, the announcer will let the class know when the group is under the judge's orders.
What happens if you knock a rail off in a hunter class?
You will automatically receive a low score. Some judges give a standard score for such a mistake, such as 40, while others will adjust it depending on the quality of the rest of the trip. The automatic low score almost guarantees that a rail will place below a clear round. A member of the ring crew will reset the jump before the next rider enters the ring.