The 22 biology students seem ready for their after-lunch nap, but Loudoun County High School's newest and youngest science teacher, Christie L. Breinig, isn't about to lose them.

She's talking about organisms and biology, note-taking skills and science journal research papers. She's doing it with obvious enthusiasm, but she's serious, too.

"This is going to help us organize our notebooks," she said.

"Just yell at me if I go too fast for you."

"I can't help you if you don't help yourself and look over [your notes]."

The sophomores and juniors are bobbing their heads from the loose-leaf binders on their desks to the blackboard as they furiously copy notes from the overhead slides.

It's her second day on the job as a first-year teacher, but at 22, Breinig exudes a been-there-done-that familiarity with the classroom, the course, the school.

That's because she has been there and done that. She was a student in the science wing less than five years ago. Now, she has returned to her alma mater to begin her professional career as a biology and chemistry instructor. It is also the school where her father, Joseph E. Breinig, is athletic director.

She is one of 35 first-year Loudoun teachers who also graduated from the fast-growing school district, said Matthew D. Britt IV, the assistant superintendent for personnel services.

"This was a great place to go to school," Breinig said in a recent interview. "And if you look at it from a professional point of view, it's a great place to be now."

She was offered teaching jobs in other school districts, including Williamsburg, and Fairfax County asked her to interview. But she had already decided on Loudoun. She said Loudoun County High offered the best professional opportunities because of the quality of the science faculty.

On Monday, the first day of school, she strolled into class with the confidence of a veteran. She calmly handed out copies of a course syllabus and outlined her expectations of her two chemistry and three biology classes.

That was Monday. On Sunday, she was a wreck. At church, her boyfriend asked how she was doing, and she replied, "Panicked."

"I think I was so terrified then that I felt better on Monday," Breinig said.

Veteran teachers have been warm and helpful, she said. But Breinig admitted that she sometimes has difficulty acknowledging the transition she's made from being their student to being their colleague.

"They say, 'Call me by my first name,' and I say, 'I can't!' " Breinig said.

When her appointment was announced at a faculty meeting in June, teachers started buzzing about her high-achieving high school career and her excitement for science.

"I think she's going to be an outstanding teacher," said Principal Edward A. Starzenski, who hired Breinig.

He said it was more than her student record that landed her a job.

"She went to a good [college] and did extremely well there. She did just an outstanding job in the interview, and she came highly recommended," he said.

Her parents also are elated to welcome her back to the family home in Leesburg.

"We're thrilled to have her back," Joe Breinig said.

He played down any influence his career might have had on his daughter's choice.

"She knows that I love what I do, but I told her to go out and go anywhere [she] wants to go. I said, 'I don't want to influence you,' " he said.

Returning to Loudoun County High wasn't always in the plans.

In 1995, Christie Breinig graduated sixth in her class, with a grade-point average of 3.96. She was a cheerleader for four years, a dancer and a member of the Latin Club and the National Honor Society. Several science teachers on campus refer to her as "my best student ever."

When she started life at the College of William and Mary in Williamsburg, she thought she was headed for medical school.

But the rigors of organic chemistry--an advanced course for premed students--persuaded her that it might not be the profession for her. By the end of her sophomore year, Breinig also decided against careers in environmental science and in private sector research laboratories and instead settled on teaching.

"It just felt right," she said.

She graduated from William and Mary in May with a major in biology and minors in chemistry and secondary education. And while some of her friends are beginning careers in big cities, at Fortune 500 companies or with fat salaries, Breinig said she couldn't be happier making $29,000 at her old school, in her home town.

"This makes me happy," she said. "If I can come home from work everyday and can be happy, that's important to me."

CAPTION: Christie L. Breinig, a Loudoun County High graduate, now teaches there.

CAPTION: Colleague Tracy Webster, left, talks with brand-new teacher Christie L. Breinig about her first day of teaching.