By the time Lexi Koontz, 14, successfully opened her locker at the end of Santa Rosa High School’s freshman orientation session, she felt ready to face her freshman year.

“Got it,” she told her mom, Kassie Koontz, after the metallic orange lock sprang open with the twist of a key. The pair had spent the previous couple hours getting Koontz’s yearbook picture taken, picking up her schedule, and locating each of the classrooms Koontz will have to navigate Wednesday, when Sonoma County’s biggest district heads back to school.

If the symbolic end of summer could be pinned to one day in Sonoma County, it would have to be Wednesday, when 23 of the county’s 40 school districts welcome back close to 33,000 students, according to state data. Districts returning that day stretch from Cloverdale Unified west to Guerneville, south to Old Adobe Union in Petaluma, and east to Kenwood.

“It kind of revitalizes you,” said Jay Meyer, a physical education teacher at Elsie Allen High School. “Teachers over the summer go into relaxation mode, and toward the end you have a feeling of, ‘I don’t know if I’m ready.’ Then you see one or two of the students you really enjoyed in class last year, and it makes you realize why you picked the profession.”

Altogether, more than three-quarters of Sonoma County schools will return this week, with school-bound cars and buses crowding roadways. Sonoma Valley Unified welcomes about 4,700 students back to school Monday. Tuesday, more than 20,000 kids will return to classrooms at Cotati-Rohnert Park Unified Middle and High schools, Oak Grove Union, Petaluma City Schools, Rincon Valley Union, Roseland and Waugh districts. Harmony Union and Monte Rio districts will return Thursday with a little over 800 students.

While the wave of enrollment crests this week, the swell actually started Aug. 6, when Santa Rosa’s Bellevue Union parents sent about 1,800 kids back to school. Then, last week, Windsor and Healdsburg Unified districts saw 7,200 students return.

Wrapping up the back to school crush, several small districts will return next week: Kashia, Horicon and Wilmar welcome back around 300 pupils Aug. 25. Fort Ross will return Aug. 26 with about 30 kids.

In higher education, Santa Rosa Junior College classes begin Monday. A little over 23,000 students had enrolled by Friday. Sonoma State University classes start Tuesday. The university is expecting its largest-ever student body, with 9,250 students enrolled for the fall term.

Last week, orientations at schools around the county helped prepare kids for their first day back.

“I thought I’d be a little more nervous, but I’m not at all,” said Koontz at the end of her session. She went to middle school in Rohnert Park and said she was looking forward to attending the larger Santa Rosa High School, where she anticipates meeting more like-minded people through the ArtQuest program, sports and other activities.

“It’s the first time she’s ever wanted to start school,” said her mother, Kassie Koontz. “She’s been talking about it all summer.”

Across campus, a group of sophomores were clustered around paperwork, comparing schedules. To them, the start of school seemed a lot less exhilarating.

“Freshman year, we were excited. This year, not so much,” said 15-year-old Bella Rabellino.

But this school year will be full of changes for students of all ages, as well as teachers. Campuses will continue rolling out Common Core, new academic standards recently adopted by the majority of U.S. states. Many districts will be focusing on math in particular, having rolled out the new language arts curriculum last year. Districts will also be implementing the computer-based Smarter Balanced Assessment, which is meant to align with Common Core standards. It will replace the paper-based Standardized Testing and Reporting (STAR) test.

At the same time, districts are adapting to a new funding model that allows them more control over how money is spent and allocates more funds to schools with disadvantaged students.

Those changes are being welcomed among many teachers and administrators.

“Never in my 18 years in the district have I seen people working so collaboratively together,” said Jason Lea, assistant superintendent of human resources at Santa Rosa City Schools.

Teachers around the county received training this summer in aspects of Common Core, which calls for incorporating more technology into education and encourages hands-on, collaborative methods of learning over rote memorization. At Santa Rosa City Schools, training included a Google session that taught instructors how to use web apps in the classroom and teacher-led presentations on how to incorporate Common Core into their various subjects.

Meyer, the Elsie Allen teacher, learned how to weave the standards into his P.E. classes.

New computers and iPads will also help schools meet the new standards, which emphasize students being literate in technology.

“We’re looking at how do we really embrace the idea of preparing kids for their future? We’re not teaching them a bucket of facts anymore,” Alexander Valley Union Superintendent Bob Raines said. “Teachers are looking at different ways to use technology in the classrooms, for kids to learn skills in a more project-based manner.”

Petaluma City Schools is purchasing Chromebooks for all its eighth-grade students, Superintendent Steve Bolman said.

“Students are very engaged when they have the opportunity to use technology in the classroom,” he said, adding that eight grade teachers have been trained over the summer on how to weave Chromebook use into their lesson plans.

Through a partnership with AT&T, the district is buying the 860 laptops at a steep discount with an anticipated price of less than $100,000.

Districts are also training and preparing for the new Smarter Balanced exam. Last spring, Sonoma County gave the test a sort of dry run, testing a little under half its students to look for kinks that needed fine-tuning before this year’s testing. That’s when the results will count.

Teachers and principals last week were busy preparing for the impending rush of students. They were awaiting the moment — an annual rite that ends weeks of anticipation — when those canary-yellow buses pull up and unload a new batch of students.

“This is crunch time,” said Cotati-Rohnert Park Unified Superintendent Robert Haley. His district is particularly busy this year as it adds an eighth-grade to Technology middle school and sees a surge in enrollment for the first time in more than a decade. “We’re looking forward to students returning,” he said. “It’s a nice life-cycle event.”

You can reach Staff Writer Jamie Hansen at 521-5205 or jamie.hansen@pressdemocrat.com. On Twitter @jamiehansen.