An "amateurishly made" pipe bomb exploded and caused minor damage to the James Fenimore Cooper Intermediate School in McLean on Wednesday right in what county fire officials believe was the latest example of a simply but dangerous kind of juvenile mischief.

The 6-inch explosive device shattered four windows and broke off parts of the outside window ledge on which it blew up shortly before 9 p.m. Although night custodians were at work in the building were 947 children attend classes during the day, no one was injured, officials said.

Investigator Nick Westhoven of the Fairfax County fire marshal's office said, "There is not much doubt in my mind" that teen-agers set off the galvanized iron pipe bomb, but there are no suspects at the moment.

Tennis players on adjacent courts reported seeing no one around the school at the time of the incident. Fire officials say they have "a couple of clues" but they refuse to say what they were.

According to Lt. Richard L. Stone of the fire marshall's office, three explosive devices went off in county schools between July, 1976, and July, 1977, and "several others" were found in schools before they were detonated."

County school officials say they have records of only four pipe-bombings in county schools since the first one exploded in Mantua Elementary School in 1972. Woodson and McLean high schools were targets of pipe bombs in 1973 and 1975, respectively. Wedensday night's explosion was the fourth pipe bombing, according to Bill Schadle, executive director of support services in the county school system.

Schadle said his information was that the other explosions probably were cherry bombs, an oval shaped illegal firecracker.

In December of last year at Arlington's Swanson Junior High School a pipe bomb caused an estimated $5,000 in damage.

Although no one has been injured in any of the bombings, the potential for tragic consequences is there, police say. Last April a janitor at National Airport was killed when he opened a box containing a pipe bomb.

In many cases, teen-agers have been seriously injured while putting together the easy-to-make explosive devices called pipe bombs, according to Stone. The ingredients for a pipe bombs can be purchased at most hardware and hobby shops or stolen from school chemistry labs, Stone said. "Most kids have the knowledge and facilities" to manufacture them, said Arlington detective P. Nielson.

Police say that the bombings usually are "just to disrupt the schools and cause mischief." The two juveniles who were found guilty of the Swanson school bombing did it as a "joke" and because of a grudge they bore against the school principal, Nielson said.

Stone said some suspects are apprehended when they return to the school to survey the damage caused by the bombing.

County fire marshals investigated 61 incidents involving explosives between July, 1976, and July, 1977, Stone said. The bulk of these (47) were caused by fireworks or cherry bombs. Many of the explosives had been placed in family mailboxes. Firing off such devices is a misdeineanor.