Ms. Grunion is a cruel, strict and pompous social worker that is attempting to get Sherman taken away from Mr. Peabody. She doesn't actually care for Sherman's well being, she actually wants to make his and Peabody's lives miserable.
In the movie
Ms. Grunion is first introduced when Mr. Peabody is meeting with the principal about Sherman biting Penny in the lunch room. She walks out of an office in the side of the Principal's office. She is shown to have more power over the principal and she is defeated by Agamemmon and the Police. In the end, as she says that one day, Mr. Peabody will make a wrong move and that then, she will get him, Agamemmon grabs her as he is taken back to his time, taking her as well.