To some extent the dress of students varies by the region and the type of program. I've seen students come to class in everything from business suits to jeans and T-shirts. For Evening programs, students often come in more formal clothes because they come directly from work and wear that type of clothing at work.
If you are uncertain about the dress code at a school, I find wearing "business casual" attire is usually a safe choice- you look professional without looking overly dressed-up.
And, of course, you can present this question to your MBA program's administration- if you have this question it is a safe assumption that others have asked it before.
Thank you for your help.