It is 2:30 p.m., but the most marketable commodity ever to play women's basketball, a curly-haired, green-eyed 18-year-old named Cheryl Miller, has to stay after school.

She must serve a half-hour's detention for being tardy four times this year to Ruth Elwell's speech class. But, as with most other things in her life, Miller sets a new standard for after-school detentions by bringing along the latest in a succession of media interviewers to serve the time with her.

It is hard for the 6-foot-2, 145-pound athlete to stay at her desk. She bubbles over with talk of her school's state championship, the senior prom, her 250 college offers, her friend's last roller-skating party, her dream of becoming the first woman to succeed in the the National Basketball Association, her opinion of UCLA's 7-foot center, Stuart Gray ("He's a sweetheart").

Finally, speech teacher Elwell, listening from the front of the room, can stand it no longer. She interrupts Miller, and the interviewer braces for a stern reprimand. "Actually," says Elwell, "Cheryl's also a very good student."

So goes life this spring for the third child of Saul Miller (6-5), the computer data manager at Riverside Community Hospital, and registered nurse Carrie Miller (5-8). It is a life seemingly blessed at every turn even before the object of all this adulation scored 105 points in a game in January and finished her high school career with 84 consecutive victories.

Cheryl Miller, the center on her team, is the only woman ever to dunk a basketball in a high school and, perhaps, any game. She finished her career at Riverside Polytechnic High School in this palm tree-lined city 56 miles east of Los Angeles with 3,446 points, 1,620 rebounds and 435 assists.

According to Mike Sondheimer, UCLA's sports information director, she is capable of taking any average college women's team "and making it a national title contender." But more important in this era of sports as business, Sondheimer said, "She is the most marketable commodity ever to hit women's basketball. Period."

The most marketable commodity is visited in her classroom detention chamber by Riverside Poly's star senior guard, Rene Overton (5-7 1/2), who has just announced she will attend San Diego State.

The two have, for once, something other than basketball on their minds. "Have you talked about. . . yet?" Overton asks Miller, making an indecipherable signal for the unspoken word. "I think they're looking for 5 or 6 o'clock." She does not mean game time, but the morning hour at which their dates to the senior prom hope to be allowed to return these two young ladies to their families.

Miller is dubious. "We got that game the next day (an AAU all-star contest in Los Angeles). I'll be lucky if he (Miller's father) lets me stay out until 3."

Greg Matthews (6-2 1/2), a sophomore basketball player at Riverside Poly, is escorting Miller, who says she has had little time for a social life in high school. But later, walking across the smoggy campus with her brother, Reggie, Miller grins hugely as a muscular athlete coming off a hard practice yells at her: "Cheryl! Your basketball career's over now that you're in love!"

At times, she says, the crush of interviews and calls from coaches take up to two to three hours a day. "The only time I get real frustrated is when I'll hang up the phone and it will immediately start ringing again," she said. "It seems as if all the coaches get together and say, 'Let's call Cheryl tonight'."

She says she likes a coach who is hard to satisfy, "constantly griping at me 24 hours a day. A lot of coaches have been intimidated by me. They wouldn't say anything to me no matter what I did. I can't stand that." In the usual drone of a coach's sales pitch, what gets her attention are the words, "I'm not sure you will be able to start."

"I like that, it's a challenge," she said.

She has narrowed her list of favored universities to eight--UCLA, USC, Cal State-Long Beach, San Diego State, Louisiana Tech, Tennessee, Kansas State and Kansas. She plans to make up her mind soon. She has visited only the four California schools, however, and says, "I'd hate to go out of state for one reason, my family."

The Millers are an unusually tight as well as tall group. Saul Miller, once a high school all-America in Memphis and a retired Air Force chief master sergeant, served as trainer for the Poly girls' team, taping ankles, videotaping every game and conducting informal backyard coaches' clinics on weekends.

Saul Jr., 24 years old and 6-2 1/2, is an Air Force sergeant and, like his father, a talented alto saxophonist. Darrell, 22 and 6-2 1/2, an all-America baseball catcher at Cal Poly-Pomona, was drafted in the third round in 1979 by the California Angels and now plays for the Holyoke, Mass., team in AA competition.

Reggie, 16 and 6-5, averaged 27 points a season for Poly's boys' team and is expected to be rushed by recruiters next year. Tammy, 13, is already 5-10 and managed her sister's team. She prefers the flute to basketball, but her father is working on her.

At UCLA, Miller was treated like a visiting head of state. Chancellor Charles Young and other campus officials introduced themselves. She toured the dormitories and spoke to professors in the theater arts department about her plans to become a news and sports television broadcaster. Her father, worried about who would tape her ankles, checked out the medical facilities.

At USC, ranked in the top 10 this year, Coach Linda Sharp was pleased that Miller was friends with the team's twin 6-3 1/2 stars, Paula and Pam McGee. But Sharp also arranged for Heisman Trophy winner Marcus Allen to escort Miller around campus.

When it came time to look at the school's broadcasting department, there was Lynn Swann, a USC graduate and Pittsburgh Steeler star, to help Miller ask the right questions. "It was an accident he was there," said Miller, then her eyes narrowed mischievously. "At least that's what they told me."

Miller will try to take her exams a week early this year so she can attend the U.S. Olympic training camp in Colorado Springs beginning June 3. She has been selected for a national women's team touring Europe this summer. The Basketball Hall of Fame in Springfield, Mass., wants the jersey she wore when she scored 105 points. She is scheduled to play in a national prep all-star game on the East Coast May 23. On May 15, Riverside will hold a Cheryl Miller Day.

She wants to make the Olympic team. Then she wants to make the NBA. Through it all, said her Riverside Poly coach, Floyd Evans, "she realizes that, after all, in 30 or 40 years people are going to say, 'Who's Cheryl Miller'?"