A 19-year-old Spingarn High School senior was sentenced to 10 years in prison yesterday for his conviction on three counts of selling cocaine, including one instance near a school.

Keith Jackson was arrested last September after being lured by DEA agents to Lafayette Square, across the street from the White House, allegedly to sell crack that President Bush used as a prop in a televised speech about the drug war.

Two juries deadlocked on that charge, once in December 1989 and again last January. But the January jury convicted Jackson of three counts of distributing drugs in other locations before the Lafayette Square sale, including a count of selling crack within 1,000 feet of a school, a crime that carries an automatic 10-year sentence.

The same jury acquitted Jackson of using a telephone to further a narcotics deal.

U.S. District Judge Stanley Sporkin told Jackson, who had no prior criminal record, that he regretted having to impose the sentence of 10 years without parole. At the same time, Sporkin urged Jackson, 19, to ask Bush for a commutation.

"He used you, in the sense of making a big drug speech," said Sporkin, the former CIA general counsel appointed to the bench by President Reagan in 1986. "But he's a decent man, a man of great compassion. Maybe he can find a way to reduce at least some of that sentence."

Sporkin told the teenager: "You have to realize, you are on the road to getting into very, very serious trouble. I think you're a nice young man and you have a future. But I do believe you were out of control for a period of time."

Jackson's lawyer, Charles Stow III, said that because of his client's publicity from the Bush speech, the authorities made an example of him.

After Jackson was pronounced guilty in January, he was taken to a holding cell adjacent to the courtroom. An observer said he was weeping and so distraught that federal marshals subdued him with a straitjacket.

Yesterday, Jackson stood calmly before Sporkin in prison garb, holding a wadded tissue. He said little except a brief statement to the judge asking for leniency. In the back row, his mother, Joyce, sobbed as his grandfather, Nathanial Jackson, made an emotional plea on his grandson's behalf.

"He didn't have a father to take care of him," the grandfather said, his voice choked. "That's why, I believe, there's trouble now."

Sporkin said that he thought 10 years was too harsh, but that, "I've got to follow the law." Congress passed the mandatory minimum law in 1988 as part of a sweeping anti-drug package. Many judges have said sentences are too stiff for first-time offenders.

"Congress has put this in the sole discretion of the government," and not the judiciary, Sporkin told Stow, who urged Sporkin to note that his client never initiated the drug sales of which he was convicted.

The Drug Enforcement Administration set up the alleged drug sale in Lafayette Square to dramatize how easy it is to buy drugs in the nation's capital. Jackson had been unknowingly in contact with a DEA informant for several months before that, according to testimony at his first trial from DEA Special Agent Sam Gaye. Gaye testified that the day before the Sept. 1 sale, his boss walked into his office and asked if he could get any of his drug suspects to make a drug sale in the 1600 block of Pennsylvania Avenue NW.

DEA officials said later that Jackson was their second choice, after the first suspect failed to show, and that the high school student had to be told where the White House was.

Assistant U.S. Attorney Mark Carroll argued yesterday against leniency for Jackson, telling Sporkin that Jackson had lied on the witness stand about his drug activities.

The three counts on which the jury convicted Jackson occurred between April and July of 1989 at two locations: Hechinger Mall in Northeast Washington and the 1900 block of East Capitol Street near Eastern High School.