Not saying it is discrimination, just that if you want a federal clerkship, your chances are practically nil at anything but a top school, and for the US supreme court, you’d better pick Harvard, Yale or Stanford. I know someone from a top DC law school who did get a federal clerkship, but it was in Tennessee and not DC or Baltimore or even her home town which has a federal courthouse and the 10th circuit.
I’ve only known one SCOTUS clerk, and he went to Harvard. SCOTUS itself is pretty much Harvard and Yale law grads, and they came up thru the federal courts. It worked for them, so they stick with what they know and hire Harvard and Yale law.