Captain Jack Sparrow is going back to school.

A London classroom got the surprise of a lifetime today, when Johnny Depp popped up for a 15 minute appearance -- in character as his "Pirates of the Caribbean" alter-ego!

According to the BBC, a 9-year-old girl named Beatrice wrote to Sparrow, and asked him to lead a mutiny against her teachers.

Don't worry -- Depp reportedly informed the girl a mutiny may get her into trouble.