From a couple of chats, my understanding of "how it all works" with respect to who owns what I create is that anything I make that is directly related to what I am being employed to do is owned by my employer. Thus, if I make an amazing mathematics lesson or worksheet or activity, etc, because I am employed to teach mathematics (in addition to other things), the school owns this. If, however, I make a fantastic third grade reading lesson it's my own work and I'm free to do with it what I like. This appears to be backed up with the analysis of Stewart McKelvey when he describes copyright as residing with me unless I create something because my employer asks me to (activities, lesson plans, etc, related to my job).
And so, looking objectively, it makes a level of sense as to why my employer owns the work I do - I'm not sure I could do this work without them, and they are, essentially, paying for it. I get to spend part of my time, either "free" or "prep", developing lessons and preparing for classes. I get to go home to a warm house, eat food, and sit down and develop various teaching ideas. Because of my salary I don't have to spend my time working at another job in order to support myself. I could, if I liked, quit my job and spend the time developing amazing teaching materials to be sold online. I know for sure, however, that I'm not willing to "take the risk" of quitting a job I love in order to develop things on my own. I also don't feel like changing professions just so I can continue working at my current one!
The bigger question, however, is should all of this be open anyway? I don't know - in my case, I'm being paid through mostly private funds. I work at a business, one with a competitive advantage based on the courses and teachers and experience it provides students. Thus, while I think freely sharing the successes and basic details of what I do, I don't think that it needs to be completely open so that another teacher or school could simply take what my school has "bought" and then re-use it. I don't believe I am even legally allowed to "copyleft" my work because, as described by the GNU Project, copyleft is really enforcing copyright but then adding additional terms to what can be shared. Since I don't own the copyright to the work I produce for school, I can't freely share it...
In the public schools, however, I think it's a different story. Because this is public money being used to pay for the various lessons and activities the teachers produce, and because saving the publics money by sharing the work being created is valuable for all, I think that, at the very least, school districts should make the work of their teachers, into an openly licensed work. Because public school teachers are also being paid for their work and thus shouldn't get to "double dip" by further selling it, it makes sense for school districts to own, but then make freely available, the work of its employees.
This all appears to go against the general culture of sharing that is promoted at professional development activities, online, through social networks or just through networking. Perhaps this is because the teachers sharing aren't making millions off the idea (usually) or, when they begin to make their millions, they tend to become private contractors anyway rather than remain in the classroom.
While my ideas may be controversial, I'm comfortable with the idea that I'm being paid a good wage to develop great lessons and activities and that I'm free, at any time, to leave this employ and develop things on my own.