I work as a secondary public school teacher in the US. I teach a fairly niche subject, that has few resources for. I am working on writing my own textbook set for this subject area. It will include a textbook, workbook, and teacher's guide. I work on this on my own time during weekends, evenings, summer holidays, etc., but the work benefits my own teaching, as I use the same material in my actual classes.
Someone warned me to check that I won't lost my rights to this work under "work for hire" or "work made for hire".
I checked my contract, district personnel handbook, site-specific handbook, and union collective bargaining documents, but find no mention of them having a right to ownership of works I create while under their employ. Is it still possible that my situation could be considered "work made for hire" under state or federal laws?
Are there any additional steps I should take to check whether I need to worry about my employer taking ownership of the work?