The kids call him "the Phantom." They say he stalks the big, modern high school on the edge of town, playing pranks on teachers, riding a stolen skateboard down air ducts and baffling police. The more imaginative among them like to believe he wears a black cape and plays the school organ late at night.

Almost every day now, the Phantom of Westminster shows up somewhere. "We heard him in social studies class this morning," Leslie Jump, 14, said yesterday. "He was in the ceiling right over me. First, he let out his low laugh, Ha-a-a-aw, Ha-a-a-aw, Ha-a-a-aw then he made this eerie noise that sounded like marble bouncing on steel."

"Our teacher, Miss Taylor, was shaking like a leaf," she added nervously."So were we. We all knew who it was."

The Phantom has been making mysterious appearances like this for more than a month at Westminster High, a huge, new $12 million school with 2,270 students in this usually peaceful town, about 25 miles north of Baltimore.

"He's playing games with us and I'm approaching my frustration threshold," complained principal Norris Weis, who confesses he has never seen anything like it in his 30 years in education.

One Sunday, for example, Weis said, "About 16 of us were having a war council on the third floor. He must have been up above us listening. Then he slipped down out of the ceiling into the next room and stole one of our rent-a-cops lunch.

"The kids love it," he added with a chuckle. "They think he's the greatest thing since ice cream."

Weis, theorizes the Phantom is a student, or former student, who has gotten hold of a set of school keys, and slips in and out of the building at will. Several people claim to have seem him; others have talked with him by phone. But all efforts to catch him - including a thorough search of the school by 20 policemen last Saturday - have failed.

"He hides in the ceiling," said Weis. "It's dark and there are acres of space up there. So it's not easy to catch him."

The Phantom delights in tantalizing school officials. Sunday night, for instance, he slipped into Weis' locked office, leaving what has become his calling card - a pile of magazines fanned out like a a hand of playing cards. At another point, he prepared a candlelight dinner in the home economics department and phoned several people to invite them to dinner.

The Phantom has never done major damage to the school, although state police have been called seven times. Each involved a report of minor vandalism - a missing tape recorder, a stolen skateboard, some broken potery in the art department, etc. Sunday, after a janitor reported someone disappearing round a corner, police brought in a dog to try to sniff out the Phantom. That didn't work. Nor did a three-hour search of the premises by 20 troopers.

"He's having a whale of a good time outsmarting us, but we'll catch him," said Weis, who finds a touch of humor in the incidents. "He's the Phantom of the Opera and at two in the morning he comes down and plays a pipe organ."

Police are baffled. "I don't think anyone is living in the school. We spent three hours searching it very thoroughly last Saturday, and I'm personally convinced no one in there," said Lt. Neil Bechtol, who is heading the state police Phantom search. "The frustrating thing is all the rumors going around the school."

Almost every student has his tale of some real or imagined encounter with the Phantom. "He really intrigues everyone," said student government president Jeff Jones, a senior. "It's the biggest thing to hit school in a long, long time."

Everything unusual that happens at the school is attributed to the Phantom. Members of an all-girls dance class, for example, reported spotting a stranger watching. When he left, they asked, "Do you suppose that was the Phantom?"

"Like everyone doesn't believe in him," commented Carla Sherfey. "But I know he's real because our class left some pretzels out for him and he ate them. He got my cousin's skateboard. He left it in speech class last week after a demonstration and it got ripped off. Then there's the girl who pased out in the bookstore. She saw his legs hanging down from the ceiling and screamed."

Jay Bogash, 14, is another believer. "I talked to him on the phone. The phone rang over there," he said, pointing to a pay booth in the school lobby. "I picked it up and he said, 'Hello there. I'm coming to get you.'

"It's all pretty neat," he added. "I want him to keep living up there. He's not hurting anyone. I like the idea of us having our own Phantom."