Mayor Joseph A. Curtatone has pledged to make school improvement the focus of his proposed budget for the next fiscal year, boosting funding for the district by more than $3 million in what he said is a bid to retain more students and increase achievement. The $171.7 spending plan adds the equivalent of 40 full-time workers by converting many part-time jobs to full-time. New initiatives include the introduction of Spanish language classes in grades 7 and 8; an intensive music program for some third- and fourth-graders; increased training for teachers; and boosts to after-school tutoring. City officials said the increases are in line with an expected influx of 500 pupils in the next decade. “We've been stable because we've made politically sensitive but smart and practical decisions to carry us through,’’ Curtatone said. “We strongly feel that investment will be driven by what we do with the education strategy.’’ The Board of Aldermen will weigh and vote on the proposal before the July 1 start of the 2013 budget year.