I will be in Paris in December and I'm trying to decide whether to go to one (or both) of the Demonstrations de l'Ecole on December 7. Could anyone please describe to me what these are like? Are these essentially classes done on stage, or are they more performance-like? There is hardly any description on the Opera website. Also, will the oldest students (graduating class) be in the 10:30am or the 2:30pm performance or both?

The demonstrations are shortenest classes done on stage. Here's the usual pattern:

Morning:
Ballet: boys and girls, 6th-4th (9-14 years old)
Mime: 5th division, boys and girls (11-13 years old)
Folk : 6th division, boys and girls (9-12 years old)
character: 4th division, boys and girls (12-14 years old)
Comtemporary: 2ond division, boys and girls (14 to 17 years old)
Choir: 6th division and 5th division

Afternoon:
Ballet: boys and girls, 3rd to 1st divisions, boys and girls (13 to 18 years old)
Adagio: 1st divisions(15-18 years old)
Character : 3rd division, boys and girls
special class: 2ond division, boys and girls

The graduating class is dancing only in the afternoon performance, but don't miss the youngest ones, the work done with them in less than a year in amazing.

But I guess you'll have a very hard time while trying to get tickets because these performances are always sold-out.

Cygneblanc, thank you very much for the information!