It's not illegal, it's just considered defacto discrimination and you have to prove it's actually beneficial.
There's no reason that you couldn't be sued the same way if you're only hiring from a certain university and that leads to excluding people from a protected class.
You can use "disparate impact" doctrine to argue that just about anything is illegal. It's pretty hard to full proof yourself against that.
But there's a big difference between something that existing legal precedent has ruled as strongly likely to be illegal for hiring (IQ testing), and a currently widespread hiring practice that may at some future date suffer the same fate (hiring from particular schools). The legal risk of the former is extremely high, while you would have to be extraordinarily unlucky to suffer damages from the latter. Even if the courts do so rule in the future, you'd have to win (lose?) the lottery and be the one or two companies that get taken to court in that test case.
There's no reason that you couldn't be sued the same way if you're only hiring from a certain university and that leads to excluding people from a protected class.