Voters will decide two contested races, for selectman and School Committee, at the polls May 21. Selectman William A. Heitin is facing a challenge from Steven E. Ross. In the School Committee race, three candidates are running for two positions: Marcy L. Kaplan is challenging incumbents Jeffrey D. Cruzan and Emily Smith-Lee. Elsewhere on the ballot, Benjamin M. Pinkowitz is running for the Planning Board; board chairman Eli Hauser is not seeking reelection after 10 years on the board. Asked about the decision, Hauser said he enjoys serving on the board, but it was time to take a break. “It’s a big commitment in terms of time and energy,” he said. Also on the ballot are Ellen Wolfson Abelson for Board of Assessors; Roberta Waterman Saphire and Alyssa Weiner Rosenbaum for two three-year positions as library trustees; Jacqueline L. Modiste for a one-year position as library trustee; and Edwin S. Little for the Housing Authority.