Enrollment is expected to reach more than 4,100 this fall at Mercer Island public schools — an increase of about one additional full classroom of students over last year.

However, the 4,146-student estimate for the coming 2012-2013 budget now being set by the school district administration is likely to be low. Budget planners at the Mercer Island School District expect that there could be even more students enrolling at Island schools this fall.

School district chief financial officer Dean Mack  said that last October, the official enrollment count came  in with 75 more students than what had been forecasted.

School officials expect the same may be true this fall.

“This is the class of the bubble,” said Gary Plano, referring to a baby boom in 2007 and 2008. “Babies born those years will be turning 5 this year, which is likely to increase enrollment,” he explained. But Plano said there is a trend that could offset that boom, where parents hold kids back a year, not entering them into school until they are 6.

About half of funding for public schools here comes from an amount paid by the state to the district on a per-student basis for what is termed ‘basic education.’ The amount to be paid out this year is $5,006.

Mack remarked that in the past, enrollment has been significantly higher than 4,000. But in 1989-1990, the number of students enrolled fell to 3,170.