The Marin County Child Care Commission mailed a letter last month asking Bolinas-Stinson Union School District board members to take measures to ensure the Stinson Beach Montessori Preschool can continue without an interruption. The privately operated preschool has rented a classroom on the Stinson campus since 2004, but the preschool’s lease is set to expire this summer after years of complaints about overcrowding from parents and teachers. The Child Care Commission’s letter claimed the preschool is necessary, providing a valuable service to 18 families “in a school district that already has a severe shortage of child care.” Aideen Gaidmore, the commission chair, cited the benefits of early care and education, saying studies have shown 80 percent of a child’s brain development occurs by age three; 90 percent, by age five. Children who attend preschool perform better in elementary school and on standardized tests, she added. “Please allow them to remain where they are, extend their lease, find them an alternative space or help them bring in a portable classroom,” Ms. Gaidmore pleaded. “Truly, children are counting on you.” Superintendent Larry Enos is drafting the district’s response. He could not be reached for comment this week while the school is in a mid-winter recess.