While searching for a certain Escher tessellation on the web I stumbled upon two great sites about M. C. Escher’s artwork and its use in the classroom:

The Mathematics behind the Art of M. C. Escher:

“As Escher’s works cover many areas from perspective distortions to metamorphoses, as such we felt that to talk a little about everything would have been explaining too little of too many things. As such, we have singled in onto one particular type of his work, tessellations.
In addition, the team felt that it we wanted to keep our explanations simple and clear, as such the extensive use of Flash animation is was chosen as the medium to relay our thoughts.
We hope to achieve through this paper:

• a deeper understanding and appreciation of Escher’s tessellations
• to uncover the underlying mathematical principles behind his artwork
• based on principles that we have learned, we will also attempt to create some original tessellations of our own
• lastly, to explore the possibilities of his artwork in practical usage in areas of architecture such as space, form and façade treatment.”

Jill Britton: Escher in the Classroom.
This is “a condensation of the article written by Jill Britton for M.C. Escher’s Legacy: A Centennial Celebration – the proceedings of the International Escher Congress held in Rome, Italy in July 1998″.

Britton also mainains a page of activity links about symmetry and tessellations.