The computer terminal at a private school here has been used to penetrate at least 21 Canadian computer systems, according to an FBI affidavit filed in U.S. District Court here.

"We don't know whether it was a little Einstein or a frustrated professor," one source said.

FBI agents with a search warrant seized two plastic bags of computer printouts and a terminal log sheet from a fifth-floor room of the expensive and private Dalton School on April 35.

The warrant was obtained after special agent Michael G. Wilson, the computer crime coordinator of the New York FBI office, told the court, "There is reason to believe that an unknown person or persons operating a terminal or terminals from the Dalton School illegally has used and is using interstate and international wire communications in furtherance of a scheme to defraud . . ."

Dalton's principal, Gardner Dunnan, said the school has no knowledge of the incident except that the FBI is investigating the use of some school telephone lines that were used to penetrate computers in Canada.

From April 16 to April 24, the school's phones were used for 42 calls to the Telenet system -- a common carrier of data communications operated by GTE Telenet Communications Corp. of Vienna, Va.

Telenet is linked to Datapac, a similar network in Canada. According to computer experts, each system has several levels of security to prevent the kind of penetration allegedly achieved from the Dalton phones, but the security methods are incompatible and thus were less effective at what is called a gateway that links the two systems.

The unknown people who penetrated Canada's computer systems found the key to access through the gateway. Then they began trying to enter various computers belonging to Canadian companies and universities.

On April 15, Canada Cement La Farge, a cement and construction company, discovered that an unauthorized person had gained access to its computer system, according to company officials.

"It was either a group trying to break our security system as a game," one company executive said. "Or my second thought was that someone was practicing on our computer to go to another system where a lot of money might be at stake."

The company's backup system restored the erased bits within 15 minuutes each time.

Canada Cement La Farbe agreed to play dumb while telephone company officials traced the calls into its computer system.

The company restricted access hours and the intruder got nastier, one executive said. "He started to kill our jobs one by one and eventually erased about a fifth of our computer -- about 10 million bits," he added.

Canada Cement La Farge officials are good-natured about the incident -- and they have increased security of their system.

"This looks like what we used to do as kids -- ring someone's doorbell and run around the block. Only, it's a more sophisticated doorbell and a longer block," and executive said.

The U.S. attorney's office and the FBI are investigating. No suspect has been named.