The Chopping Block regularly hosts their classes/demonstrations at Fox & Obel
401 E. Illinois
Chicago, IL, 60611
Also try the Culinary School at the;
The Illinois Institute of Art - Chicago
350 N. Orleans St.
Chicago, Illinois 60654
1 (312) 280-3500
1 (800) 351-345
Lexington College - www.lexingtoncollege.edu
310 S Peoria St # 512
I hosted a corporate event at the Marshall Field's (it'll just never be Macy's) State Street Culinary Institute. It was great, they allow you to bring your own wines and work with the chosen faculty chef to design a menu. We did a taste of Tuscany for about 30 people and it was great.
The Culinary Department at Kendall College has a wonderful demonstration kitchen, with classroom seating (arranged amphitheater style) for perhaps 100 people. They may even have more than one demonstration kitchen, but I know they have that one and it's a terrific setup. It's not right downtown, but very close:
900 North Branch Street (West of Halsted Street, North of Chicago Avenue)