Public officials who deprive a person of constitutional rights without a hearing must pay him substantial damages if he proves he's been harmed, but only nominal damages - up to $1 - if he doesn't, the Supreme Court ruled, 8 to 0, yesterday.

Nominal damages are a recognition by the law of "the importance to organized society that those rights be scrupulously observed," Justice Lewis F. Powell Jr. wrote in the opinion for the court.

But the law also "remains true to the principle that substantial damages should be awarded only to compensate actual injury," Powell said.

The court acted in a Chicago case in which public schools suspended two students without providing the procedural safeguards required by the Constitution for a deprivation of life, liberty or property by a state.

One of the students, Jarius Piphus, was suspended by the principal of Chicago Vocational High School of smoking marijuana in violation of a rule against the use of drugs. Piphus claimed to be innocent. The principal suspended him for 20 days.

The other student, sixth-grader Silas Brisco, wore an earring one day at Clara Barton Elementary School. A year before, the principal had issued a rule against the wearing of earrings by male students, in the belief that earrings denoted membership in terror-prone street gangs.

Brisco, asserting that the earring symbolized black pride, not gang membership, refused to remove the earring. The principal suspended him also for 20 days.

Both suspendions were curtailed by court orders after the students' parents sued under a civil rights law enacted in the Reconstruction era to implement the guarantee of due process of law in the 14th Amendment. Piphus sought $3,000, Brisco $5,000.

A federal judge ruled that Piphus and Brisco had been suspended without procedural due process, but had not established that they were injured and therefore could not collect damages from defendant school officials.

The 7th U.S. Circuit Court of Appeals sent the case back. It ruled that even if the judge were to find that the suspensions had been justified, and even if the students hadn't suffered an injury, such as mental suffering or emotional anguish, they would be entitled to recover substantial non-punitive damages because they had been denied procedural due process.

Reversing the appeals court, the Supreme Court rejected any contention "that damages should be presumed to flow from every deprivation of procedural due process."

Powell pointed out that "a person may not even know that procedures were deficient until he enlists the aid of counsel to challenge a perceived substantive deprivation."

Justice Thurgood Marshall concurred only in the judgment. Justice Harry A. Blackmun, who was ill when the case was considered, did not participate.

The court handed down decisions in two cases: JURY SIZE

The Constitution guarantees a trial by jury to a person accused of a crime. In 1970, the court ruled that in a state criminal trial, a six-person jury was sufficient. Yesterday, the court decided unanimously that a jury of fewer than six persons is insufficient.

The case involved the Criminal Court of Fulton County in Atlanta, which, under a Georgia law, had been trying persons with five jurors since 1891 (although it switched to six two years ago).

Claude D. Ballew, arrested in 1973 for showing "Behind the Green Door" in an adult film theater, appeared before a jury of five, moved for a panel of 12, [WORD ILLEGIBLE], and in Georgia appellate courts, lost again.

The decision to overturn the lower courts was announced by Justice Blackmun. POISONOUS "FRUIT"

The court ruled, 6 to 2 in a New York state gambling case that in a trial for perjury committed after an illegal search, the prosecution could use the uncoerced testimony of a witness whose identity or whose possession of relevant information was learned as a result of the search.

In the opinion for the court, Justice William H. Rehnquist wrote that the link between the search and the testimony was too weak to justify strict application of the "exclusionary rule," which bars, as "fruit of the poisonous tree," evidence from an illegal search.