Well not exactly. There are groupings within even the top schools. And students from schools like Yale and Stanford tend to funnel into prestigious judicial clerkships.
For example, you're going to need roughly the same percentile rank to get a biglaw job from Michigan and Virginia even though one might be ranked higher than the other in any given year.
Plus firms like diversity in their classes, Biglaw firm X in NYC might rather hire, say 5 students each from UM, UVA, and Penn, even though UM has a larger class and there are more students sitting at X percentile rank that Biglaw firm X requires than at the other two schools. And students from say, Berkeley might prefer to take a job at a firm that pays biglaw money, but only has west coast offices, which means those students don't fit the definition of what many people think is biglaw.
I think once you get past UCLA/USC/Vandy/Texas, the rankings become pretty mushy. For example, does WashU really place better than GW? I doubt it, but in recent years WashU has been higher ranked. Schools like W&L, W&M, Minnesota, etc. have never had great biglaw placement. Yet those are so-called top 30 schools. Ranking doesn't equate with placement.
Check this out for a very rough idea of how biglaw placement works. Note that when the data the legal job market was in a boom. Or at least the tail end of a boom.http://taxprof.typepad.com/taxprof_blog/2008/04/law-school-rank.html
Here is another interesting link. Note that this is for the class of 2005.http://pdfserver.amlaw.com/nlj/20080414employment_trends.pdf
Basically, in a good economy, if you want a very good chance at a biglaw job, you're best shot is to attend a top 10 school, or preferably, a top 5 school.
Just keep in mind that right now all bets are off for biglaw no matter where you go unless the economy substantially improves. And outside of biglaw, salaries drop off considerably. Depending on what state or region you are in, $30-50K at a small firm might be considered "good" if you can find a legal job at all. For the next few years, you are going to see a lot of fresh law grads doing quasi-legal or non-legal work for low pay. Many will be lucky to have health insurance and a salaried position. That's even students from top 20 schools.