It looks like the top 10% from 6th to 8th grade Society of Science fairs are invited to participate. They are then selected down to a top 300[1] and a top 30.[2] You can find a project name for the top 300 and a paragraph on each of the top 30.

[1] https://www.societyforscience.org/jic/2025-top-300-junior-in... [2] https://www.societyforscience.org/jic/2025-finalists/