As parents, educators and city leaders gathered at a Roxbury school Saturday morning to talk about revamping Boston’s school choice system, the conversation kept turning to the sometimes dramatic inequalities among the city’s schools. “Until we make every school equal, then we can talk about changing the school assignment process,’’ said Karen Kast-McBride, a Roslindale parent who has two children in the system. Interest in neighborhood schools - and the notion that families sharing a nearby school might build a stronger community - as fueled a new discussion about changing the school assignment process.