The Prince William County School Board decided last night to add an 11th grade to a new high school scheduled to open next fall, if enough 10th-graders who now attend Osbourn Park High indicate they want to attend the new school.

A proposal to open the new school on Spriggs Road -- as yet unnamed -- with only the ninth and 10th grades generated objections at two public hearings last week.

Last night, the board voted to add the 11th grade if 150 students from Osbourn Park want to transfer. That is about the number of 10th-graders at Osbourn Park who indicated in a survey released by the board this week that they want to attend the new school.

The original plan was for adding 11th and 12th grades over the next two years, but many parents said their children would gladly go to the new school and give up long bus rides.

Too few students who will be seniors next year responded to the survey to make a 12th grade feasible next year, officials said.

The board also decided to study the possibility of making Osbourn Park High an arts and humanities center open to students from throughout the county. That proposal, which would take effect in 1992, will come up for a vote in April.

The board voted to permit students now attending Featherstone and Rippon Elementary schools to choose which school to attend. Some parents sued the board for changing the two schools' attendance boundaries last year, claiming that the board acted capriciously.