Here's the link to the data:
http://www.law.com/jsp/nlj/PubArticleNL ... 2543436520
Also, since the percentages fluctuate from year to year, I made a spreadsheet with the data for the last three year, along with a mean and rankings.

For people new to TLS: the numbers above are the percentages of each class that end up at a firm in the NLJ 250, which is a ranking of the 250 largest law firms in the US. The NLJ 250 is a crude proxy for 'Big Law.' It doesn't include people who get clerkships and then go to Big Law after that (over 10% of the class at some schools).