Boston schools Superintendent Carol R. Johnson and the School Committee are treading cautiously as they weigh how to overhaul the way students are assigned to schools, a hot-button issue that has polarized parents, community activists, and political leaders in the past. “I think that for us to have a very well-designed process and thoughtful communication we need to make sure we take the time so we are prepared,’’ Johnson said. “It’s a very complex issue.’’