Hi,
I've finaly solved the problem, and I give the solution here if anyone has to meet this requierment:
In the ACS, you must use 2 groups, one group for the instructors, and the other group for the students. In the Student group setting, you tick the "Shell (exec)" box and the "Auto command" box, adding the exec command you want to perform after students logged (in my case it's the "menu" command with the string "console" as a parameter).
In the instructors group you tick the "Shell (exec)" box only.
And the you will configure AAA authorization in order to permit the students to run the exec Automand, with the command (under line vty):
authorization exec LIST
where LIST is your authorization list.
The config of the Terminal Server (cisco 2600) and the view of the ACS is attached.
Bye !