Joi Jefferson never will forget her very first day at Riverdale Baptist High School, because that was the day the school's gym was destroyed by a fire.

Jefferson had chosen to attend the Upper Marlboro private school for a variety of reasons, both academic and religious, but her primary motivation was finding a school where she could play basketball right away. Needless to say, the fire concerned Jefferson.

"I thought, 'What have I gotten myself into?' " Jefferson said. "I had some doubts."

Four years later, Jefferson said she is certain she made the right choice. Her team is undefeated and ranked No. 1 by The Post for the first time in school history. The school's boys basketball team has its highest ranking ever (10th). And both teams have a nice, bright gym they can call home.

"The basketball teams have created some excitement here," said Terry Terrill, the athletic director at the 250-student high school. "This has sort of been in the works. [Boys coach Walter] Webb and [girls coach Keith] Lynch have done a good job."

Several factors have contributed to Riverdale Baptist's success: the enthusiasm of the coaches--Webb, who is in his fifth year as the boys varsity head coach, and Lynch, the second-year girls varsity head coach--the addition of talented transfers, and perseverance and patience.

"It's a plus for any school to have success in both [the boys and girls basketball] programs," Webb said.

"Our baseball [team] does well, and that's three solid programs at the school. It helps the school and how people perceive the school."

"As a private school, if you don't offer some worthwhile programs or something special--whether it's discipline, athletics, or academics--why would you pay money to go to the school?" said Terrill, who also has coached Riverdale Baptist's nationally ranked baseball team for 24 years. "We don't tell our coaches to shoot for second place. The goal in life is to win, and the essence of a winner is more than just the final score. It's how you act, how you carry yourself."

When Jefferson was a freshman, "we took our lumps," she said.

The Crusaders played some of the area's top teams, such as Largo, and lost--badly. But Riverdale Baptist steadily improved and was ranked for the first time last season.

"I told our coaches last year that we were one year away from being considered one of the top teams in the area," Lynch said. "Our slogan this year, which we say before the game in the locker room, is, 'This is our year.' "

This season, the Riverdale Baptist girls team won the prestigious IAABO tournament, beating Largo in the championship game. It was the Crusaders' first win over the Maryland 4A power.

The Crusaders won their first 15 games this season by an average margin of 50 points. Three players--Jefferson, Jennifer Baker and Vicky Briscoe--already have signed with Division I schools, and three more players also are expected to sign.

"Every year they seem to pick up a quality person in addition to the quality kids they already have," said DuVal girls coach Walter Clark, whose 18th-ranked Tigers have lost to Riverdale Baptist twice this season. "This year, Tiffany [Stansbury] augmented what was already a good team, with players who had been together for four years."

Lynch stressed that eight of the 11 players on the girls varsity team have attended Riverdale Baptist for at least three years, saying, "I want to kill the perception that we heavily recruit players." The core of this year's team--seniors Jefferson, Baker and Sarah Jenkins--have been at the school since they were freshmen, before Lynch joined the team as an assistant coach three seasons ago.

Senior Terrelle Waller and junior Dawn Sims transferred from Northwestern before last season, and Stansbury joined the team this season from Philadelphia.

Stansbury, a 6-foot-3 senior center who was named the most valuable player at the IAABO tournament, said she learned about Riverdale Baptist from a George Mason University coach who was recruiting her. The coach introduced her to Lynch.

"I wanted to find a better environment, where I could focus on my grades," said Stansbury, who lives with Jenkins's family. "I liked the positive atmosphere at the school. But it was a very hard decision, because I left all my friends and family in Philly."

The boys team also is a mix of longtime Riverdale Baptist students and recent transfers. Senior Wayne Gibson, who broke the 2,000-point mark earlier this season, and junior Ryan Terrill, the athletic director's son, have attended Riverdale Baptist since kindergarten.

Last season, junior Greg Harrison transferred from McNamara and senior Patrick Atangana came to the school from Cameroon.

This season, junior Emmanuel Weatherspoon transferred from McNamara and senior Kevin Bell transferred from Manhattan's Brother Rice, which finished the 1998-99 season ranked No. 2 in the country.

"I wanted to get away from home," said Bell, who lives with a cousin in Arlington and commutes 45 minutes to the school via Metro.

"There were a lot of kids on the team [at Rice] who were good, and I felt overshadowed. Coach Webb had seen me play and he knew people that I knew. I wanted to focus on school and go to a good program where I could score or distribute the ball."

Like the girls, the boys team is enjoying its best season.

The Crusaders were 19-3 heading into Monday's game with No. 5 Montrose Christian. Last year's team finished 37-13; the previous season, Riverdale Baptist was 22-21.

"When I was in sixth grade, the basketball program was not good at all," Gibson said. "People laughed at it. Other teams played their reserves, not their starters, when they played us. Coach Webb changed the program around and brought in players who could get it done.

"This year we have come together."

Now that the school's gym has been rebuilt (it reopened last January), the teams finally have a place to play in front of their fans. Both teams had to be creative while the gym was being rebuilt: The boys team occasionally would practice in classrooms; the girls team would pile into a van and travel to churches and recreation centers to practice. Every game was a road game.

"The gym situation made you extremely tired, because you were always traveling back and forth to practice or games," said Raven Mallard, a junior forward who starts for the girls team.

"It's a luxury to have a gym. This is the first year that a lot of students have come out to watch us. We're established in the public eye.

"I love coming back to school and having people talk about our games."

CAPTION: Riverdale Baptist boys basketball coach Walter Webb, above, and girls coach Keith Lynch have made their teams two of the best in D.C. area.

CAPTION: Riverdale Baptist, led by Terrelle Walker (left), makes its way to the court for a recent game. Vincent Briscoe and Bessie Jefferson, who have daughters on the team, lead the cheers.