Los Angeles school board members have voted to spend up to $18 million to retain current teacher staffing at schools with declining enrollment, a move that Supt. David L. Brewer says could lead to financial trouble. The vote came amid pressure from the teachers union and community groups, which have argued that a midyear reshuffling of classrooms and teachers unacceptably disrupts thousands of students. The looming teacher reduction was among several budget-balancing measures adopted earlier this year at Brewer's recommendation. But those were approved by the previous school board. A new majority, endorsed by Mayor Antonio Villaraigosa, took control in July and has added expenses and tried to undo key cuts in the $6.2-billion budget.