[nixme]

Is this a form of discrimination?

It is a form of exclusion based on past education and experience. Are you implying that it is unethical to do so? I say no. Past performance is the best predictor of future performance. Getting accepted into and performing well at a top-tier university is past performance. This will undoubtedly exclude many great candidates. But the reality of hiring is that there's always a time constraint and bad hires are incredibly disastrous to an organization.

Is it legal?

In the United States, it is certainly legal to judge candidates on their education and experience. I'd also wager it's legal just about everywhere. Otherwise, as tsally said, employers would be forced to interview just about anyone that applies for a position.