Negima!? Magister Negi Magi Episode 13: Rather Than a Question of You Being the Enemy, the Issue Is Really Whether or Not You Are a Nuisance to the Master.
Episode 13 of 26 of Negima!? Magister Negi Magi, aired December 27, 2006, 25 minutes. Available with English subtitles and dub where released.
Episode summary
All of Class 3-A has found out about Negi's secret about being a mage and because of this, Negi is punished by being transformed into a chupacabra. Even worse is that Class 3-A has been transported to the World of Illusions. How will Class 3-A get out of this situation?