For the second time in three weeks yesterday some students at Walt Whitman High School in Montgomery County protested drug arrests by county police adjacent to the school grounds. The protest included blocking traffic, throwing rocks at a police car and turning in false fire alarms, according to police and school officials.

The protest began about 11:30 a.m. - a lunch period - after police arrested six youths, aged 14 to 17, on marijuana possession charges in a parking lot near the school.

School authorities said about 100 students stood in the middle of Whittier Boulevard and interrupted traffic for about five minutes. Student leaders estimated that only about 20 students were involved and the others were bystanders.

Police said no one was injured when some students threw rocks at a police car.

School authorities also said from 20 to 40 students marched in protest of the arrests to the Area One school administration office at the Whittier Woods Elementary School next door, broke some bottles against tha side of the buildings and went inside, shouting in the halls for about five minutes.

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At least three false fire alarms were sounded at Whitman during the afternoon.

Police said two of the students arrested near Whitman yesterday attend Whitman and the other four said they attend a private school in Glen Echo.

Following drug arrests near Whitman on Sept. 27, about 400 students blocked traffic, surrounded a police car and impeded the movement of school buses.

In their continuing crackdown on drug use at county schools, Montgomery police also arrested three students, aged 16 to 19, yesterday in the Senecca Valley High School parking lot on marijuana possession charges.