Mitchell (Stop, Thief!) or Mitchell (Let's Kill Hitler) as this is the name given in Stop, Thief!
Talk about it here.
She once taught about the sinking of the Titanic in 1912, and asked Mels why it occurred. She answered that the Titanic sank because the Doctor didn't save it, and called her teacher stupid for not knowing about him. Mels was sent to the headteacher's office for this comment. (TV: Let's Kill Hitler) At some point, she also assigned a task for them to write about their imaginary friends. Rory wrote about The Chiropractor, Amelia wrote about the Raggedy Doctor, and Mels wrote about someone who tells her to kill people. (PROSE: Stop, Thief)