A trickle began this week, but the real tsunami of back-to-school waves hits Monday as students in Sonoma County's largest school district say goodbye to summer and head back into the classroom.

In addition to the more than 16,500 students expected to attend Santa Rosa City Schools classrooms, students in Bellevue, Mark West, Twin Hills and Wilmar are heading back. The fall semester at Santa Rosa Junior College also begins Monday.

"I'm nervous and happy," incoming Santa Rosa Middle School seventh-grader Alex Reynolds said of his move to middle school.

"Just being in a new school, that makes everybody nervous," he said, standing in line for his class schedule Friday. "But I'm just excited to go to a new school and find new friends and experience new things."

Cotati-Rohnert Park School District and Sonoma State University begin classes Tuesday, and the biggest wave of returning kindergarten through 12th graders will be Wednesday when Petaluma, Rincon Valley, Roseland, West Sonoma County, Oak Grove, Old Adobe and others go back to class.

"The tone around here is as positive as I've ever seen it," said Healdsburg Superintendent Jeff Harding.

The 1,900-student district, which started class on Thursday, was able to add back five classroom days for the 2012-2013 school year, bringing the calendar back to 180 days.

Yet budget insecurity remains a dominant theme in many of the county's 40 school districts this year.

Officials are watching closely the outcome of Gov. Jerry Brown's tax initiative on the November ballot because passage could stave off further cuts.

Brown's initiative proposes raising income tax rates on a sliding scale on incomes of more than $250,000 for seven years and increasing the state sales tax by a quarter percentage point for four years to help ease state budget woes. If voters reject it, the Legislature could impose about $6 billion in automatic cuts, primarily to schools and higher education.

Districts were directed to submit budgets for the current year based on the assumption that the initiative will fail. Many districts already are precariously close to not being able to provide balanced budgets two and three years into the future, as required by the state.

"Almost everybody is of concern because of the potential of those tax initiatives not to pass," said Denise Calvert, deputy superintendent of business services for the county Office of Education.

Santa Rosa school officials predict cuts of $13 million in 2013-2014 if the initiative fails.

"There is definitely trepidation," said Andy Brennan, president of the Santa Rosa Teachers Association.

The district already has sliced six days from its calendar, which has meant a 3.25 percent pay cut to teachers for the current year, Brennan said.

Other changes in the district include the opening of Santa Rosa French American Charter School at the former Doyle Park Elementary School, which was closed in May. More than 260 students are enrolled at the French language school.

Ninety-six 96 former Doyle students who will remain on the campus but as a satellite campus of nearby Brook Hill Elementary School.

In Bellevue District, the former Kawana Elementary School will open its doors Monday as Kawana Academy of Arts & Sciences Charter school under new Principal Carol Castro.

In Mark West, two of the district's three elementary schools will become charters when doors open Monday. San Miguel is focusing on a global theme, and Riebli will focus on science, math, engineering and technology, Superintendent Ron Calloway said.

The move to charters also allows schools to accept transfers from outside district boundaries without approval of the student's home district. It also can draw more per-pupil funding from the state in some cases.

"It was a way of capturing a little funding for us," Calloway said.

For incoming Santa Rosa Middle School seventh-grader Angel Maravilla, budget woes and shortened schedule were not weighing heavy on his mind as he wandered the E Street campus Friday in preparation for the first day of class Monday.

"I feel kind of nervous," he said. "But I'm excited because everybody says seventh grade is one of the best years you have in school."

(Staff Writer Kerry Benefield writes an education blog at extracredit.blogs.pressdemocrat.com. She can be reached at 526-8671, kerry.benefield@pressdemocrat.com or on Twitter @benefield)