The Queen star is slated to provide the "inner voice" of one of the characters, reports TVLine.com.
A source tells the website, "Helen came in and secretly recorded several long and hilarious monologues two weeks ago. Then she visited the kids on set, and they had no idea she was coming; they were stunned. Helen's a big fan of the show and the message it sends out about arts education."
The episode featuring Mirren is said to air in America next month (Jan12).
The New Year will be a star-studded one for Glee - Gloria Estefan and Ricky Martin were recently confirmed to join the cast for a Latin-themed show.