I would agree with John -- the teacher would release responsibility and the "heavy lifting" to the students, giving guidelines on how to demonstrate mastery but ensuring students own the process. I would guess that there's a balance of student choice and teacher coaching to figure out the best form of demonstrating advanced knowledge.
Great question! Here's a response from Jon Hanover, their founder & executive director:
We currently only go through 3rd grade and so have a couple years to figure out the middle school question for our scholars :) The schools I'd point them do at the secondary level are probably the same ones you would - Summit, High Tech High, etc. If by local they mean Denver, DSISD is a high school that adopted the PLP last year and there is a new high school starting next fall called the Cube that is thinking similarly.