The Copyright Act of 1976 states that materials teachers make within the scope of their employment in a school district are considered "work for hire" and belong to the school system. However, I think if a teacher makes lessons on their own time and resources (such as a resource they made to be sold on Teachers Pay Teachers) those should be considered the teacher's intellectual property, Not the school's. Hence, They should be allowed to sell it or give it away freely, If they choose to do so.
Teachers are usually idiots. Their lesson plans are public property and not even important. They don't own anything they create as they are public servants and should be treated as such. They are payed by tax money and should shut up and be greatful that they got a job. Without public funding they would starve.