Superintendents, experts worry about the loss of veteran teacher knowledge.

When students return Thursday for the first day of school across Wisconsin, many familiar faces will be gone, as teachers chose retirement over coming back in the wake of a new law that forces them to pay more for benefits while taking away most of their collective bargaining rights.

Documents obtained by the Associated Press under the state’s open records law show that about twice as many public school teachers decided to hang it up in the first half of this year as in each of the past two full years, part of a mass exit of public employees.

Their departures came before the new law took effect, changes pushed by Gov. Scott Walker and the Republican Legislature that led to weeks of protests at the Capitol.

The ensuing exodus of teachers and other state employees has led to fears that the jobs might not be filled, and that classroom leadership by veteran teachers will be lost.

Ginny Fleck, a German teacher from Green Bay with 30 years of experience, is among nearly 5,000 teachers who retired.

“It wouldn’t make sense for me to teach one more year and basically lose $8,000,” she said. Fleck, 69, decided to retire in February, even before the bill became law, in part because of the hit she would take to her $60,000 annual salary, and because of other changes the district was making.

In the first six months of 2011, overall public employee retirements were double that in all of either 2009 or 2010, according to data provided to the AP by the Wisconsin Retirement System. That includes 4,935 Wisconsin school district employees who started receiving retirement benefits, up from 2,527 teacher retirements in all of 2010 and 2,417 in 2009.

Teachers weren’t the only ones heading for the exits.