Washington (CNN) -- A new report on high school course work indicates that students are completing more challenging courses and taking more credits than they did in the past.
The High School Transcript Study looked at the curriculum choices for the 2009 graduating class at more than 700 public and private schools representing 37,700 students from across the U.S.
According to the study, graduates in 2009 earned three more credits than their counterparts in 1990, amounting to an additional 420 hours of instruction over four years of high school per student. They also took more challenging courses.
The study divided the students into three curriculum levels: standard, midlevel and rigorous. A rigorous designation indicated the student completed four credits of English, three credits in social studies, four credits in mathematics that included geometry, algebra and precalculus or higher, three credits in science that included biology, chemistry and physics and at least three foreign language credits.
Thirteen percent of the high school graduates in 2009 took the courses required to rank in the rigorous category. This is up from 5% in 1990.
Students who took Algebra I before high school were much more likely to reach the rigorous level according to the study.
"There is a correlation, but not causation," said Henry Kranendonk, who is a member of the National Assessment Governing Board. "The correlation is clearly strong that students who are taking algebra prior to high school are setting themselves up for a more rigorous curriculum."
The mathematics requirements to reach the rigorous benchmark are hard to reach in four years of high school so a head start helps, he added.
Just adding algebra to a middle school curriculum is not necessarily the answer.
"The current certification for teachers in the middle school level, by and large, is not adequate for them to be teaching the kinds of courses we are talking about, like algebra and geometry," Kranendonk said.
Back in 1990, 60% of students completed less than the standard curriculum requirements. That number dropped to 25% in 2009.
For the study, a standard designation meant that the student completed four credit hours in English and three each in social studies, mathematics and science.
The lack of a science course was generally the reason students did not make a higher designation.
"Many graduates lacked only the required science courses to reach the next higher curriculum level," according to Janis Brown, a statistician for the Department of Education. "For example, of the 25% of graduates who did not complete a standard curriculum, 39% lacked only the required three credits in science to attain a standard curriculum."
The High School Transcript Study is done every four or five years by the National Assessment of Educational Progress to look at trends in coursework, credits and grades of U.S. high school graduates.
The full report can be found at http://www.nagb.org/high-school-transcript/.