At least 21 persons, including D.C. School Board member Frank Shaffer-Corona, who were arrested at night or in late afternoon by D.C. police this year did not spend the full night in jail, as do ordinary suspects.

Instead, city judges, using a little known informal procedure, ordered them released almost immediately with a telephone call to police.

The releases were made under a discretionary exercise or judicial power that was criticized severely by the city's judicial conduct commission four years ago. The commission said its use raised an "appearance of impropriety" and was "ill-advised."

The key to successful use of the system is knowing a judge -- or knowing a lawyer of other intermediary with access to a judge -- who is willing to order police by telephone to release a suspect before the next day's normal court hours.

The system, which has no written rules and is not part of official court policy despite a suggestion four years ago by the D.C. Superior Court chief judge that regulations be established, has been used in a variety of cases ranging from suspects charged with homicide to persons picked up on traffic charges.

Last Saturday, in a predawn phone call, Superior Court Judge William S. Thompson ordered Shaffer-Corona freed on personal recognizance after he was arrested on a charge of assaulting a police officer. Other defendants arrested that night had to wait several more hours until midmorning Saturday for a formal court proceeding at which bail was set.

Thompson said he ordered Shaffer-Corona's release over the telephone after being contacted by School Board President Barabara Lett Simmons, whom he knows personally. He said he ordered the release because he was certain Shaffer-Corona would appear for arraignment as he later did.

Thompson said he had a vague recollection that another Superior Court judge was criticized for a similar act several years ago, but said he was aware of no court rule or procedure controlling such releases.

Several lawyers and other members of the criminal justice system here acknowledge that judges have the authority to release defendants with telephone calls and that such a system should be available, but few were aware that the system is unregulated.

"It looks like there is a mechanism in the system that allows judges to do favors for friends," said one lawyer who asked not to be identified. "It amounts to a secret night court system."

There have been various proposals for a night court system here to insure that defendants arrested in the late afternoon or at night are given a prompt court hearing. However, those proposals have never been put into effect, largely because of cost and manpower shortages.

Shaffer-Corona said he was "concerned" that he was given treatment that was not afforded most other arrested persons but said he was just as concerned about what he considers the mistreatment he suffered from the police when he was arrested.

Shaffer-Corona was arrested and charged with assaulting a police officer in a dispute with police that broke out while the school board member was having a 3 a.m. cup of coffee at a restaurant on Columbia Road NW. He has denied the charges and claims he was brutalized by the police.

The police department, which has a special procedure for assuring that it is a judge -- and not an impostor -- that actually calls and orders a suspect released, keeps an internal record of such releases. It shows that judges called approximately 13 times in 1979 to release defendants and approximately 18 times the year before that.

A federal magistrate on one occasion this year called in to release a group of eight antiabortion demonstrators who had been arrested in a holiday period when the courts were closed.

Another time, the police department itself initiated the release process in a homicide case and called a judge at home. Police had arrested and charged a District government employe in a late night shooting at the District Building, but were confident her self-defense claim in the death would cause the case to be later dropped by prosecutors. The judge apparently agreed, and she was ordered released that night.

A police official said a handful of "aware citizens" such as Shaffer-Corona know how to use the system, while thousands of other defendants arrested here know nothing of the emergency system.

The D.C. Commission on Judicial Disabilities and Tenure publicly chastised D.C. Superior Court Judge Edward A. Beard in 1976 for ordering an artist friends released from jail.

At the time, commission chairman Henry A. Berliner Jr. said, "The overall trend is against . . . justice handed out on the basis of who you know, who your lawyer is, what your connection is."

There is at least one designated emergency judge each night available by telephone for Superior Court matters, and the judicial tenure commission said Beard should have referred the release at that time to one of those judges. He did not have the emergency duty on the night he ordered his friend's release.

After the Beard incident, then D.C. Superior Court Judge Harold H. Greene told the other judges in a memo that such services should not be provided on the basis of personal favoritism and that it should be "sparingly exercised" only by the designated emergency judge.

No official policy was ever set by the full court, however, according to Acting D.C. Superior Court Chief Judge Fred McIntyre this week.

"As I understand now, any judge" can order a defendant released if the judge is assured the person will show up in court later, McIntyre said.