Construction started Aug. 17 on the new Marshfield High School, according to Bob Brait, a principal and project manager at Brait Builders, the general contractor. Brait said workers spent the first few days building a temporary road for school buses. The football stadium, tennis courts, and track must be demolished to make way for the new building, which will be built adjacent to the old. The Marshfield-based company’s winning bid of just under $85 million came in about $2 million less than the estimated cost, he said, and he expects to include “alternate” items that the town could have cut if costs ran too high, such as a stadium and track. Town Meeting approved $103.9 million over the course of two meetings last year to cover design, construction, and other costs.