"The math courses that students take in 9th grade are a strong predictor of the highest level of math they will take in high school, according to the 2005 NAEP high school transcript study. Ninth graders who complete Algebra I or Geometry are far more likely to take Algebra II or more advanced math, such as Calculus, in high school -- subjects that generally cover content and skills that professors and employers say are needed for college and work. These results reinforce the importance of strong preparation in middle school to ensure that students are ready for a rigorous high school curriculum."
Source: U.S. Department of Education, Institute of Education Sciences, National Center for Education Statistics, High School Transcript Study (HSTS), 2005.
Some more math tidbits:
Getting beyond Algebra 2 in high school increases the likelihood a student will earn a bachelor's degree!
Highest high school math / Percent who earned bachelor's degree
Calculus (9.7%) 83.3%
Precalculus (10.8%) 74.6%
Trigonometry (12.1%) 60%
Algebra 2 (30%) 39.3%
Geometry (14.2%) 16.7%
Algebra I (16.5%) 7.0%
Pre-algebra (6.7%) 3.9%
Source: 2006 report by Clifford Adleman, U.S. Dept of Education
Purdue University in Indiana announced in 2010 that applicants must have 4 years of math. "Studies show that the higher the level of math in high school, the more likely students are to succeed in and graduate from college."