The Prince William County School Board raised high school graduation requirements last night and created the first college preparatory diploma in Northern Virginia.

Beginning next year with the class of 1987, county students must complete 20 credits of study rather than 18 credits in order to graduate. A credit is awarded for one year of study.

Most Northern Virginia school systems already require 20 credits for high school graduation, and the Virginia Board of Education, which sets minimum state standards, is considering a 20-credit requirement. The state now requires only 18 credits.

Prince William students completing a rigorous 22-credit course of study that includes three credits each in a foreign language, math and science will receive a special college preparatory diploma.

School spokeswoman Kristy Larson said the board action on graduation requirements, which met with no community opposition, is in anticipation of changes in requirements at the state level.

The board actions were recommended by a task force, which reported that many colleges consider high school graduates unprepared for higher study. The report of the task force said a college preparatory course of study will strengthen the value of the diploma in the eyes of college admissions officers.

Last week, Prince George's County raised the number of math and science courses students must take for graduation.