Rather than ordering Castilleja to summarily slash headcount from the current 448 to the permissible maximum of 415, Turner said the school may reduce attendance "through natural attrition and voluntary measures, such as acceptance of fewer new incoming students."
In addition, it must pay the fine by Oct. 25 and enforce a traffic-reduction program that will be verified by the city in unannounced visits, Turner said.
Castilleja's Kauffman said the school this fall launched a morning shuttle service that serves 40 to 60 students from Woodside and Los Altos, added parking spaces on campus, and dedicated staff members to monitoring school traffic in the morning and throughout the day.
"Castilleja School takes full responsibility for its nonconformance and for exceeding the enrollment limit," Kauffman said Monday.
"Going forward, the school remains committed to working with the City of Palo Alto to resolve the nonconformance. At the same time, the school will continue to implement transportation demand management (TDM) programs, designed to mitigate the parking and traffic issues in and around the neighborhood."
Kauffman said the school will confer with city officials to work out details of its plans to reduce traffic and attendance.
Turner said the city's fine against Castilleja was calculated at a charge of $500 a day for the 600 school days since 2010 that the school has been in violation of its enrollment cap. Though the violation goes back to the 2002-03 school year, "The city is only permitted to go back three years in assessing penalties," he said.
Castilleja's enrollment cap of 415 dates back to a "conditional use permit" issued by the city in 2000, when the school sought to grow beyond what was then its maximum of 325 girls. The use permit — which the school obeyed for the first two years — came with a host of conditions, including that the school hold twice-yearly neighborhood meetings and maintain a transportation-management plan.
Kauffman has said enrollment crept up gradually over the years as more girls than anticipated accepted Castilleja's offers of admission. Kauffman announced in July she would seek a new growth cap of 515 but has backed off from that plan pending resolution of the current violation. It was at that time that she said she realized the school was out of compliance and told neighbors and city officials about the problem.
Kauffman has said the school needs to grow slightly to gather a critical mass of girls to participate in cutting-edge programs in subjects like digital fabrication, robotics and Mandarin, which she said are important offerings for 21st-century learning.
In addition, she said, there's been intense demand for spots at the school, where tuition is more than $36,000 a year and about 20 percent of students get tuition assistance.
Stan Shore, who has lived across Kellogg Avenue from Castilleja for more than 20 years, praised Turner and the city staff for taking the time to "craft a comprehensive enforcement plan" to address the school's violations.
"It appears that during the first full month of the school year the school is doing an outstanding job of mitigating traffic and parking problems," Shore said Monday.
"If Castilleja had been doing what they are now doing for the preceding 10 years, a significant portion of the previous years' traffic and parking problems would have never happened."
At the same time, Shore said: "No one should lose sight of the root cause of all the parking and traffic problem — too many students — resulting in too many cars for this quiet residential neighborhood."
Shore said he would have preferred a doubling of the city's fine of Castilleja — from $500 a day to $1,000 a day — to give the school "additional motivation to reduce enrollment to the agreed on 415 students."
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