People aren't stupid.
Diversity and Affirmative Action in general:
People aren't stupid. If certain groups or individuals were admitted due to relaxed standards, classmates can tell quickly. In the course of my undergraduate study I probably met 500+ people who I could identify by name. After meeting enough people, the mind begins to classify - and, yes, stereotype - people, both consciously and unconsciously. The brain is a wonderfully sensitive signal processing machine.
Affirmative action fools no one.