Sarah Schneider / 90.5 WESA The first public debate between candidates for the Pittsburgh Public Schools Wednesday evening was a relatively sedate affair, made only mildly contentious by the fact that three of the candidates have children who attended charter schools. Three incumbents and eight challengers took part in the three-hour-long forum, which was hosted by a coalition of progressive groups and the Pittsburgh Federation of Teachers. Only District 9 incumbent Veronica Edwards did not participate. Over the course of the evening, the 11 candidates frequently acknowledged they were echoing each other on many subjects. But a discussion of charters — public schools that receive public tax dollars but operate independently of the city district’s administration and board — provided a rare moment of contention.