The turnover saved the district an estimated $4.9 million and brought in fresh minds and undimmed enthusiasm, school officials said. It also caused a union dispute and required an enormous amount of training. 

The mass exodus started in 1997 when state officials determined that the Pennsylvania teacher pension fund, which had invested in a soaring stock market, was awash in money. It offered all public school teachers who'd served at least 30 years the chance to retire early without losing benefits (the normal term is 35 years). 

Many Bucks districts lost dozens of teachers, and Centennial was no different. It had 93 retirements - many of them early. That number represented almost one-quarter of the district's teaching staff of 450. Now, more than half of its staff has been there less than 10 years. 

Replacing all those teachers in one year was difficult. The district filled 95 percent of the vacant jobs. Administrators, board members and the teachers themselves say the district now has the perfect blend of youth and experience. 

"We've lost an awful lot of years of experience," said district human relations coordinator Jean Rollo. "But we're gaining people with enthusiasm that comes from being new, and people who know the current modes of thought of education."

Young teachers fresh out of school are generally more familiar with new teaching techniques and technologies, which is important in a world where educational philosophies change often. For example, Centennial has been trying to institute new statewide standards and high school block scheduling, and teachers who started last year will accept them as routine. 

The downside is the inexperience of the new teachers. State law mandates that each new teacher be mentored by an experienced teacher, and Centennial teachers are mentored weekly. 

But the last time teachers negotiated a new contract, they thought they weren't paid enough for their new mentoring duties. Now, the district hires outside mentors - largely retired administrators. 

Lawrence Belli, principal of Davis Elementary School in Upper Southampton, said it takes about three years to get a new teacher up to speed. 

At Upper Southampton's Stackpole Elementary School, the third-grade classrooms of Rob Rothman and Shirley Smith are adjacent, luckily for mentor Smith and new teacher Rothman. 

It's Rothman's second year at Stackpole, although he has taught before at a private school and as a substitute. He said last year - his first - was 10 months of daily joy and constant bewilderment. 

"I would not know what to do without Shirley," Rothman said. "The first time we had early dismissal, I didn't know what time to let the kids go. The information wasn't anywhere. I just ran next door and asked her." 

Now in his second year now and loving it, Rothman can share pointers with the six other new teachers at the school. 

"I've been fortunate to hire people that are very bright and talented," said Stackpole Principal Pearline Greene. "They've been well-received by parents and teachers. It's been very smooth." 

The district's finance department also likes the change.

The financial benefits have been astounding, and will continue to help the school board for the next few years as it struggles to avoid raising property taxes in Upper Southampton, Warminster and Ivyland officials said. 

The difference in salary and benefits between the top and the bottom of the teacher pay scale is about $45,000 a year. That sum, multiplied by 110 teachers, equals $4.9 million a year in the district's annual budget of $55 million. There are also savings on insurance and Social Security taxes.

"It's been positive for everybody: the financial benefits, the operational benefits, the benefits to the staff," said school board member Eugene Grossi.