A recent test showed extremely high levels of lead at Frederick Douglass Elementary School in North Philadelphia, but officials didn’t tell parents until reporters with WHYY’s Keystone Crossroads and PlanPhilly started asking. It turns out the school probably isn’t alone, but Avi Wolfman-Arent and Ryan Briggs explain why it’s hard to say just how many other older school buildings in the region could have similar serious problems with their water.