Montgomery County public school students' scores on nationally administered, standardized tests have reached a seven-year high for grades three, five and seven, according to figures released Monday by Superintendent Edward Andrews.

But ninth graders' scores indicated no improvement in arithmetic and a decline in reading skills since the tests were first given in Montgomery County in 1972.

Third, fifth and seventh graders showed an increase in both reading and arithmetic skills over the seven-year period.

However, Montgomery County students in all four grades scored higher for this year than students did nationwide.

Third, fifth, seventh and ninth graders throughout the state have taken the Cognitive Abilities Test (CAT) and Iowa Test of Basic Skills (ITBS) annually since 1972 to measure general verbal and nonverbal aptitudes and 11 specific academic skills such as reading, arithmetic concepts, graph reading and punctuation.

Based on the test publisher's 1970 norms:

Third graders scored a median of 71 percent in reading and 69 percent in math, with an overall average (based on the 11 ITBS tests) of 79.

Fifth graders scored 64 percent in reading, 72 percent in math and 74 percent overall.

Seventh graders scored 56 percent in reading, 66 percent in math and 66 percent overall.

Ninth graders scored 55 percent in reading, 58 percent in math and 62 percent overall.

"These tests have nothing to do with grades and nothing to do with getting into college," said Dr. Kenneth Muir, a school system spokesman. "Possibly, there is less motivation for the ninth graders to perform at their best."

"Student achievement tests are one important indicator of how well the school system is performing," said Superintendent Andrews. "Public schools in this county are doing a good job."

Andrews said attendance, discipline and the strength of alternative programs are other important indicators of how a school system is performing. He said the test results will be used to plan new instructional programs.

"I am pleased, but not satisfied," noted Andrews. "We still have a lot more to do."

The 1970-80 school year was the last one in which Maryland students will take the Iowa tests. They will switch to another series, the California Achievement Test Battery, which the state board of education decided reflects more closely the Maryland public school's curriculum.

School officials are working on methods of comparing results of the new test with those compiled from the Iowa tests.