I interned for a summer at a large non-profit firm that is quite well known in NYC (e.g. it has many government contacts and gets sponsorship from a lot of BigLaw firms). Throughout the summer program, brown bag lunches were held periodically to give the attorneys and directors of various units a chance to discuss life in the public sector. At one of these meetings, the director of the Litigation Unit said that positions in non-profit are very competitive because there aren't many to begin with, and specifically that a good GPA from a good law school is something they definitely take note of.
In general, I think a larger portion of graduates from top law schools end up in the private sector, but if they were to consider the public sector, their credentials would give them a leg up.
Agreed. Sexy PI jobs are extremely competitive (especially in metro areas) and the employers do take a school's reputation into account. I will say this: if your class rank is good enough to get you into a big firm, and you still opt for PI, employers will take note of that as well. But look at the staff attorneys for the ACLU, for example. They all went to T1 (and in many cases, Top 14) schools.