GPA and LSAT scores are the most important factors for law school admission, but at the most elite Law Schools, the tiebreaker often is the rigor of the undergraduate program.
That is why tiny Amherst College has 18 of its graduates at Yale Law School right now, while very good state flagships that are 10-20 times larger have one or two (or usually zero) of their students at Yale. It can’t be just because Amherst students do better on the LSAT. On the average they do, but large state schools have thousands of students with qualifications that might have made them competitive for Amherst. Amherst, with a graduating class of 450 students a year, has more of its graduates currently at Yale Law than the entire Southeastern Conference (with well over 400,000 college students).
http://bulletin.printer.yale.edu/htmlfiles/law/law-school-students.html
I admit that I think this only holds true for the very top handful of law schools, where some tiebreaker is necessary because of the sheer number of applicants with top GPAs and LSAT scores.