This is one of my favorite ideas and I'm glad it was moved to the Evolve stage. Encouraging greater student input will lead to higher levels of engagement. Teachers would need to understand the need of flexibility of the 20% time based and grade level, subject etc.; adjusting accordingly.
Since teaching and instructions tend to be so focused on standards and prescribed learning goals, one way to help ensure teacher buy in is to show teachers have to connect student's selected learning topics/ ideas with required standards.
Provide some structure and routine; limited,not to stifle.
Determine how the 20% (+/-) fits in to mandated instructional minutes. The time could be added to already scheduled activities and routines such as daily journal writing, "Do Now", exit tickets.