Oakland and Alameda schoolchildren said goodbye to summer and returned to the classroom Monday to meet new classmates, new teachers and -- in some cases -- new principals.

Many schools have their own special back-to-school traditions, but it's hard to imagine more fanfare than that Monday morning at Grass Valley Elementary, a school in the hills above the Oakland Zoo.

PTA members with pompons flanked the front lobby, cheering on the children and their parents the moment they walked through the double doors. Members of the Grass Valley Dads Club greeted families and recruited members outside, on their way to or from the school.

"A lot of kids are new," explained Mildred Oliver, the school's PTA president, who is also an Oakland police officer. "A lot of kids are coming in with some kind of anxiety, and we just want to welcome them."

For some Bay Area teachers, the new school year doesn't just mean different students; it means more of them. Class sizes, which once averaged 20 students in kindergarten through third grade, have risen in recent years. Some districts now average 30 students or more in those grades. At Grass Valley Elementary, Helen Behr led a game so that her 28 first-graders could practice walking around safely without bumping into one another.

In a back-to-school Facebook post to her Oakland colleagues, Oakland teacher Ilyse Opas wrote that the roster for her first- and second-grade combination class included 32 students.

"32 is the new 20!" she wrote. "Enjoy, y'all."

Despite the hundreds of pink slips issued in March and the dozens of final layoff notices delivered in May, the Oakland school district ended up offering jobs to its laid-off K-12 teachers -- and hired 195 more, said Jeffrey Dillon, the district's recruitment supervisor. He attributed the open positions to high teacher turnover. As of Monday, he said, there were two vacancies in special education, but that the other classroom teaching positions had been filled.

About 18 of Oakland's roughly 100 schools has a new principal this year.

Early reports suggested a smooth start to the school year -- in most places. Students on the Castlemont high school campus and at East Oakland PRIDE Elementary school, however, spent hours in an emergency lockdown as police searched the area for suspects in a carjacking and armed confrontation with officers that had taken place across town earlier in the day.

The police-ordered lockdown at Castlemont began just a few minutes after Oakland Superintendent Tony Smith had finished his back-to-school tour of the MacArthur Boulevard campus, about 12:30 p.m., district spokesman Troy Flint said. Flint, who spent the afternoon in the Castlemont cafeteria because of the lockdown, said it lasted for nearly three hours.

Julissa Lambert-Yank, a teacher on special assignment at East Oakland PRIDE, said because the lockdown happened at lunchtime, some children were forced to remain in the cafeteria for hours, while others waited as long to eat lunch.

"We're very saddened by the whole thing," she said. Tomorrow, she said, "will be a retake."

Read Katy Murphy's Oakland schools blog at www.IBAbuzz.com/education. Follow her at Twitter.com/KatyMurphy.