D.C. School Superintendent Floretta D. McKenzie said yesterday she was surprised when as many as 100 high school students admitted using drugs when asked during an address by the Rev. Jesse Jackson on Thursday, though she praised the students' candor as "the first step" in seeking help.

Jackson surprised some 900 students gathered for an assembly at Spingarn High School in Northeast when he asked those who had experimented with, or were on drugs, to come to the front of the room.

McKenzie said school officials have not yet determined how to handle the students who responded to Jackson's call, but that her office and the school's prinicipal are working on a plan.

"Those students were strong enough to admit it and we plan to help them," she said. "We plan to start by building on the drug abuse prevention program that already exists at Spingarn."

McKenzie said she could not assess whether the response at Spingarn indicated drug problems at other city schools, but that "this city has a very big drug problem and apparently so do our schools."

Jerome Leonard, 20, who said he was a Spingarn senior and one of the students who answered Jackson's call and signed up at the school to get help, told a reporter yesterday that he has been "smoking and drinking" since he was 12 and has tried to quit both "more than once" but failed.

Spingarn Principal Clemmie Strayhorn said he, too, was "very surprised" to have so many of his students admit using drugs. "We are in the business of helping out students," he said. "But we are not sure yet what strategies we are going to take."

Jackson, visiting Spingarn Thursday to congratulate the school's basketball team, the Green Wave, on winning the local interhigh school championship, also had encouraged team members to challenge President Reagan, with whom they were scheduled to meet yesterday at the White House. Jackson suggested they ask Reagan about cutbacks in student aid and government scholarships.

During yesterday's brief Rose Garden ceremony, Reagan congratulated the team, then preempted any potential queries from them by saying that Jackson "didn't quite understand our position regarding aid to education. We're not in favor of cutting it back. We're in favor of redirecting it."

None of the ballplayers or school officials responded to Reagan's comment.

In a separate development yesterday, D.C. school board member R. Calvin Lockridge (Ward 8) asked for an investigation into allegations that some school employes were selling drugs to students.

Lockridge, who targeted Anacostia and Ballou high schools as the center of the problem, said, "There have been recent reports of the open distribution of illegal substances in our schools, by school employes, on an ongoing basis."

McKenzie said she would take "prompt action" but said she had no proof of any of the allegations and has requested more information from Lockridge.

Meanwhile, Spingarn student Leonard, waiting outside the White House and hoping to see his schoolmates honored, said he was the first to sign his name on the list of students who admitted using drugs.

"I felt proud to sign my name on that list . . . . That was something that I needed.

"I had a bumper [full glass] of Champale today and I may have something later," he said, "but I wouldn't have told the truth to anyone else except Jackson . . . because I think the man really cares."

Leonard said he knows it is his responsibility to confront the problem. "If I'm the one who has to quit, then I'm the one who needs to make the decision.

"But I do need help!"