Making lesson plans a work-for-hire is only going to make them more restricted. As it is now, most teachers are happy to share their work for free, and when they move from school to school or district to district, they are able to bring what works best for them with them. Once you start letting schools (or districts) consider this stuff proprietary/copyrighted information belonging to the school, they'll end up wreaking havoc (or at least trying to) trying to protect their interests whenever a teacher leaves their district, or helps someone at a conference by "giving away" the school's intellectual property.
It seems moot anyway. IANAL, but copyright law leaves a specific exemption for educational purposes. A teacher can copy whatever the hell he or she wishes for use in the classroom, and that seems like it would include lesson plans. I could see making your own plans available online and charging a small convenience/thank you fee for them, but if another teacher gets ahold of your lesson plans through some other means I don't see what recourse you would have anyway.