Ogion, the system is not Mexico-wide. The NAUM does it because have an autonomous state of existence, so the government educational proposes - laws doesnt really matter to them. Most of Mexico's schools and Universities are ruled by the SEP, wich is the "Secretary of Public Education", and even private schools need to use SEP's educational program (a.k.a the classes you got in a certain career, etc), the only institutions wich dont need to do it are the autonomous universities, there is one per state and the national one.
And I think it works, I have the poorest and the richest people i'v met in my life as classmates, and its actually nice because I havent ever seen an act of discrimination, ever.
Also the NAUM does 70% of Mexico's national research and all mexico's nobel prizes come from here, I mean, I know that Mexico is far from being a educational superpower but I think NAUM is doing a nice job, and in my opinion the way they select their students has to do a lot with it. And maybe the massive budget too :p