Hardin Coleman, dean of Boston University’s School of Education, has been appointed to the Boston School Committee, Mayor Thomas M. Menino announced Thursday. Coleman will take over the seat once held by John Barros, who stepped down in April to run for mayor. Coleman’s stay on the committee could be brief. The term is set to expire on Jan. 6, and if Coleman wants to continue on the committee, he would have to seek reappointment. Coleman joins the committee at a critical time. The seven-member board is about to begin a search for a new superintendent to replace Carol R. Johnson, who will retire this summer.