I don’t think it is necessarily discrimination in the negative way we think about discriminating by race, SES status, religion, etc… It is just an efficient screening mechanism, the assumption being that if you are in the top quarter/third of a highly regarded law school, you have cleared a pretty high initial hurdle. For example, when I recruited, I interviewed at Stanford and Berkeley only in NorCal. There are other fine law schools in the Bay Area, but there are limitations on time and money for recruiting. Similarly, I interviewed at UCLA and USC only in SoCal. We spread a wider net in NYC, interviewing beyond Columbia and NYU at schools like Cardozo and Fordham.
The first job is pretty key for your second job. If you are an associate at one of the top BigLaw firms, it is easier to get a lateral job at another firm, including another prestigious BigLaw firm, go in house for some Fortune 500 or even jump industry (eg IB). If your job is at a lesser firm, it is very hard to move up in prestige (and compensation) unless you have developed a particular specialty.