Kids at 40 schools this morning participated in San Francisco’s biggest Bike to School Day yet. The level of participation, in the event’s fourth year, reflects steady growth in levels of biking and walking to schools throughout the year.

San Francisco Bicycle Coalition (SFBC) Executive Director Leah Shahum said an enthusiastic group of more than 50 kids rolled in on two “bike trains” at this morning’s ride to Sunnyside Elementary School, despite pouring rain.

The citywide attendance numbers aren’t in yet, but organizers anticipated about 2,000 participants this year – a sharp rise since the city’s first event in 2009, which saw about 600. Participation has steadily grown since. Last year, 1,600 students turned out for Bike to School Day, including 120 at Grattan Elementary in Cole Valley — one-third of the school’s students, Shahum pointed out.

Biking to school throughout the rest of the year is also becoming more popular among students and parents with the help of San Francisco’s Safe Routes to School (SRTS) program, which began promoting walking and biking at 15 schools in October 2009.

In May, Sunnyside Elementary will finish up its popular Walk and Roll Wednesdays, which offers kids prizes for walking, biking, or taking transit to school. Near Glen Park, Fairmount Elementary also holds multiple bike trains every Tuesday. SRTS staff also teach biking skills at the participating schools and provide family education along with the SFBC, which just released a revamped version of its Family Biking Guide.

Bike to School Day also seems to be attracting more city officials each year. SF School Board President Norman Yee rode in to Sunnyside Elementary this morning, and five city supervisors biked to schools in their respective districts: Carmen Chu, Eric Mar, John Avalos, Christina Olague, and Jane Kim.

Parents say the event increasingly serves as an “entry into riding” for many students, said Shahum, who noted that 42 percent of elementary school students live within one mile of their school, according to data from the SF Unified School District (SFUSD). The district is also making it a higher priority to place students at schools within their neighborhoods, and the SFMTA is currently implementing 15 MPH zones at over 200 schools around the city.

“What we’re seeing more and more, especially among [SRTS schools] is more parents, teachers, and school leaders engage well beyond Bike to School Day,” she said.

Nik Kaestner, director of sustainability for SFUSD, said the bike racks “were overflowing” at Sunnyside this morning. To meet bike parking demand, SFUSD is close to installing up to four bike racks (which hold eight bikes each) at all 104 of the schools in the district, he said.

“We’ve put in the infrastructure that makes it easier for parents to leave the bike there during the day instead of having to schlep it back,” said Kaestner. SFUSD will also evaluate the usage of the racks at each school to determine which schools need more capacity. High schools, he noted, are particularly likely candidates.

“It definitely seems like biking is something that is in and hip right now in San Francisco, and our parents want their kids to be part of that culture,” he said. “We’re seeing that when we go to the schools that participate.”