A global answer to the question how to license free educational content is: apply a GFDL on it.
However there are some complications. http://people.debian.org/~srivasta/Position_Statement.xhtml points this out.
Alternatives are multiple licenses and a contract to give us the rights in order to defend interests.
Our preferred content license will be the GNU FDL, but other OSI approved licenses can be used too (but not mixed). Public domain material is also acceptable ofcourse. To keep in mind: diverting to different licenses won't help us, yes even harms us.
Working in the spirit of Free Software is a must. A social contract like http://www.debian.org/social_contract.en.html can help stressing it.
A standard question for educational institutions/authors should be: please relicense some of the material for free use use. The motto: "Give a little, take a lot".