Don't forget your helmet...that is a must at any English riding show...
If you have paddock boots and half chaps they should be more than adequate for a schooling show. Paddock boots with jodphurs or breeches that stay tucked inside your boot will work too...(try to wear socks the same color as your riding pant so if they should slide up it isn't so noticeable)
A riding jacket... do you have a dark colored blazer, no pattern or thin pinstripes...will do in a pinch if they require, but for a schooling show... riding jacket is probably not a requirement.
Have fun and enjoy!