Teachers selling their lesson plans online has become a booming business, driven by higher standards and other teachers willing to pay out of their own pockets for classroom-tested material.

Top sites include Teachwise, Teacher's Notebook and Teachers Pay Teachers, which says it has helped make millionaires out of about a dozen of its contributors since launching a decade ago.

But some educators worry that the increasing monetizing of lesson plans will stifle the longstanding practice of teachers freely sharing their ideas. And legal experts question whether teachers actually have ownership of the lessons they are offering.

For teachers buying the materials, however, it's a huge time saver, allowing them to reclaim the nights and weekends otherwise spent starting lessons from scratch.