Suppose a group of high-school students broke into the computer network of the Internal Revenue Service and wrote their own tax-rebate checks. Is it possible? Well, it hasn't happened yet.

But with the recent publicity about youthful electronic burglars using computers to gain access to confidential data, the Treasury Department, which houses a vast computer network, is a little nervous.

Enter Richard Shriver. The government's only computer director at the level of assistant secretary, Shriver has been charged with updating the department's antiquated computer system, which, among other things, is responsible for transferring President Reagan's paychecks into his bank account every other week.

It clearly wouldn't do to have the president's paycheck pilfered. Nor does the department welcome outside tinkering with any of the 200 million electronic transactions a year it controls.

"We have sponsored several joint government and industry discussions on electronic funds transfer crime and computer crime," said Shriver, who has held the job eight months. "When we're moving 200 million transactions a year, that's probably about $200 billion."

Although a lot of recent concern has focused on youngsters or other computer enthusiasts accidentally penetrating so-called confidential computer systems, Shriver said the department needs its safeguards because "a lot of this type of thing is done by insiders."

Shriver said he didn't know of any successful attempt to penetrate the funds-transfer system.

One of the major concerns is the IRS, a Treasury wing, which has the largest data-processing operation in the government outside the Defense Department, Shriver said.

"It's a very well protected system," he said. But there is always the danger that someone can enter the system and write their own check or alter a tax rebate or other check intended for them or someone else, Shriver said.

Treasury has recently discussed the problem in special forums with other agencies such as the Secret Service, Bureau of Government Financial Operations, Comptroller of the Currency and the Justice Department as well as private bank-security organizations and insurance companies, Shriver said.

Shriver said the insurance industry wrote its first computer break-in policy in 1981 and this year the industry plans to write several hundred.

Shriver meets periodically with Deputy Treasury Secretary R.T. McNamar to discuss the different departments' computer systems and how they are functioning, but his responsibilities go well beyond acting as an in-house silicon security guard.

He also is in charge of improving management of telecommunications activities, such as maintaining the privacy of classified radio or telephone transmissions, and he is concerned with upgrading cash management to improve the outflow and receipt of funds.

The government disburses $900 billion a year and collects between $600 billion and $700 billion. "In the next few years, just improving management of cash will save several billion in interest on the public debt," Shriver said.

As far as high-level support goes, well, let's just say that Shriver's boss, Treasury Secretary Donald T. Regan, is a computer enthusiast.

"Regan has a computer terminal on his desk," Shriver said. "He uses it to get current information or economic data and market prices." Regan already receives one news service, and Shriver said he soon will be able to call up stories from certain newspapers.