At the 2014 Technology Exchange in Indianapolis, Valter Nordh presented the approach that The GÉANT project is taking on assisting NRENs (normally smaller) that yet don't have a federation.
The technical solution also works for K12.
See the webinar on Federation as a Service at:
https://www.youtube.com/watch?v=QAcb-Tm3ocY&list=PLELuOn8jN3IK3UY9zVLjuPY6byW_tP_0A
See Slide Presentation on Federation as a Service (PDF)
Read about K12 Federation in Sweden (this site is in Swedish)
Policy template for federations