Do I have any grounds for a lawsuit for invasion of privacy or harassment?

The Fair Labor Standards Act was recently amended to require that employers provide a reasonable break time for employees to express breast milk for nursing for one year after the child's birth. The employer is required to provide a place (other than a bathroom) that is shielded from view and free from intrusion from coworkers and/or the public. I would recommend that you consult with an attorney to further discuss your rights under this Act and any possible causes of action that you might have against your employer.

Probably not. Especially with the rash of incidents where teachers have engaged in inappropriate conduct with students in their classrooms. You may have a case, if you had previously spoken to administration and gotten approval to have privacy in your classroom to pump, then perhaps you may have a case. The school's need to protect students and ensure wrongful conduct is not occurring would trump privacy rights. You should speak to administration if you have not and see about arranging a time and place for you to go and have privacy to do this during the day. I am confident that your school system has a policy for this or has dealt with it before. You may have a case for invasion of privacy only if you have a right to privacy, which you don't have in school. The only way you would have had this is if administration said that you could close off your room to pump. These facts do not give rise for a cause of action for Harassment, unless you can show that the Superintendent was aware of the agreement for you to have privacy and came in the room on purpose to see you pumping.

I know of no remedy. You do not own the school property. While that was rude, I do not see any legal violation. The law is not intended to correct every wounded feeling. You may want to contact an attorney that handles such matters.

How did your superintendent know you were pumping breast milk? Remember, he or she is responsible for the school and it may behoove him or her to know what requires such privacy. What if you were molesting a young man? Then would it be an invasion of privacy? Also, if the superintendent were a woman, would you have the same feeling? No, I don't think you have claim for invasion of privacy. You may be lucky to still have a job if the district has a policy that expressing breast milk should be done in the female teacher's lavatory.

These facts will not support a lawsuit.

Outside of an agreement between yourself and the school district to the contrary, you have no expectation of privacy in your classroom, regardless of the sign you put up. Based upon your post I do not believe you have a cause of action.