If you want to literally write a book that you will both use in your own class and sell, you should frame the task to say "I'm writing a book to sell, and I will also use it in my class!" rather than "I would now like to sell this book which I wrote as part of my job." I'm not saying this is airtight legal advice, but it couldn't hurt.
I really don't think lesson plans strictly defined -- the plan for how to combine different external content, activities, etc. -- are strongly copyrightable. Ideas are not copyrightable.
Specific content you create, images, photos, PowerPoints, online lessons, expressly for use in your classroom, I don't see how it isn't the property of your employer.
I'm not really making an argument in favor of the "work for hire" approach -- I'm just describing the current law as I understand it.