A rash of incidents, describes as graduation-eve pranks, broke out in eight Mongtgomery high schools yesterday as more than 9,000 graduating seniors celebrated the next to last day of class.

"They do some dumb things like this every year," said Lt. Leonard King, spokesman for the county fire department, which was kept busy all day responding to trash fires, bomb threats and false alarms.

In the most serious incident, 21 students and two teachers at Montgomery Blair High School were hospitalized with watering eyes and breathing difficulties after a World War II-vintage tear gas bomb exploded shortly before 1 p.m. in a second floor corridor.

The students and teachers were treated at Holy Cross and Washington adventist hospitals and released after gas permeated 25 classrooms of Building C at the Silver Spring high school.

"These things come under the general heading of senior pranks," said school spokesman Pally Ondrasik. "Unfortunately we tend to see this thing happen year after year."

The day of pranks began when police discovered that a pipe-bomb had exploded some time Wednesday night on an outside window sill at Bethesda-Chevy Chase High School. The blast knocked out two windows in the school's administrative offices.

At Richard Montgomery a trash fire was set before noon and 12:40 the school was evacuated after a bomb threat was called in. The fire was out by the time firefighters arrived, and no bomb was found.

At 8:12 at Rockville High School, three trash cans were set ablaze, but the fire died before firemen arrived. Trash under the playing field bleachers at Woodward High School was torched at 12:37 and went out on its own. A small fire was set at Wootten High School, and false alarms were rung at Wheaton and Einstein high schools.

Earlier this week at Churchill High School, 15 pranksters carried a 2-ton gazebo from a nearby intersection and deposited it on the high school grounds, not far from where pranksters in the Class of '79 had felled a 50-year-old oak tree.

"Kids are kids," Ondrasik said. "They think it's funny at the time."