What is the average size for a junior high discus?

There isn't a standardized "junior high discus" size. The size of a discus used in track and field competitions depends on the age and gender of the athlete, and these categories generally don't align with the "junior high" school grade levels which vary slightly by location.

To find the appropriate discus size, you need to look at the governing body's rules for the specific age group competing (e.g., USATF, IAAF/World Athletics, etc.). You'll find different weights and diameters for different age groups, likely ranging from lighter weights for younger junior high athletes to heavier weights for older or more advanced ones.