About a quarter of Maryland's 55,000 11th graders have not yet passed a statewide writing test mandatory for graduation, prompting education officials to boost the number of chances to take the test from two to three, Maryland Superintendent of Education David Hornbeck said today.

Hornbeck also told the state Board of Education, meeting here today, that he plans to hire an independent panel of experts to evaulate the two-part essay, which has come under attack by some parents and some educators as too tough.

Members of this year's junior class are the first in the state to be required to pass proficiency tests in reading, writing and math before being allowed to graduate.

Students also will have to pass a citizenship test starting in 1988. They are allowed to take the tests once a year beginning in the ninth grade until they pass.

Hornbeck said that about 98 percent of the 11th graders have passed the reading test and that more than 80 percent have passed the math test.

Ordinarily this year's 11th graders were to receive two more chances to take the writing test, in April and in January 1987. But Hornbeck said the test will be offered to them next September as well. He said he will ask local school systems to offer summer remedial programs in writing to prepare students for the test.

Although Hornbeck defended the test and the way it is scored, he admitted there is reason to be concerned about the high number of students who are not passing. Students are asked to write two essays. Each essay is graded on a scale of 1 to 4 by two persons. The scores are averaged for a final score. The highest score is an 8 and passing is 5.5.

"We have youngsters in the state who have over 10 and 11 years received As and Bs and they have not passed the test, and that is a source of consternation and concern and even anger on the part of their parents," Hornbeck said.

At the same time, he added, he is not ready to give up on the test, and believes eventually it will do what it is supposed to. "I think we'll see kids writing and thinking in ways we haven't seen before," he said.

However, he added that he would reconsider whether to continue re-quiring the writing test a year from now if 13,700 students, nearly a quarter of the class, still have not passed. "I would find that intolerable. I would reach the conclusion that we had not done a proper job of teaching them" how to write, Hornbeck said.

Several board members said they are mainly concerned by parents' complaints that the scoring system is not fair. Maryland has a contract with a private company in Georgia, R&R Evaluations, to score the test.

"The issue is objectivity," said board member Albertine Lancaster. "That's the bottom line."

But state board President Frederick Schoenbrodt cautioned against scrapping the test because of complaints about the way it is being scored. "I don't think any one of us wants to throw out the baby with the bath water," he said. "It's just a matter of adjusting the temperature."