A cc licence is just a per se license, not the only ever doable thing with it.
at the same time offering commercial licencing f.e. through getty
images.
Like others, I am doing commercial trainings and in this regard would
not give away extended material on a pure 'commercial usable' license
when this in fact is my income. In this regard you will need to decide
what kind of system you want to have, f.e. all classes documented and
updated on a Attribution-ShareAlike licence by volunteers all the time.
This also might collide with existing contracts and others,
preventing people from using material they already have
to use in this when this is a mandatory commercial usage
license.
I assume that stockholm is referenced as having donations
for the event? that is not a commercial use of the material.
Please note also that while CC is great, there are different
issues with usage rights depending on where something is
release, by german law for example I cannot give up my
copyright and always have the right to repel any use
of my material as a creator - no matter what the licence
i tried to give with cc.
Summary: I would not dismiss a non commercial option
for the general material. Once somebody approaches
you for commercial use, you can always go back
to the creator and discuss further options.
Take it the other way round: You make good materials
and release it as just Attribution-ShareAlike, I can
take those and make commercial training out
of that without the need to give anything back.
I am painting a black and white picture here, the solution
is not that of course, but this is not a quick simple
decision. :)