I just realized the job I quit a few weeks ago has not taken me off their payroll. Can I get in legal trouble for not telling them?

I worked in an American public school through the end of last school year. I just received a pay check for this school year even though I have not been working there. I imagine they will realize pretty quickly that they made a mistake, but am I in any way obligated to pay them back? Can they file a legal claim against me?