An apparent loophole in Pennsylvania’s mask mandate for schools is making it easier for some students to go to class without having to cover their faces, even as state education regulators sought to make an example of one openly defiant school board. The state health secretary’s order requiring masks to be worn inside K-12 school and child care facilities, which took effect Tuesday, includes an exemption for students who claim it would cause or worsen a medical condition. But there’s no obligation in the masking order for a student to produce a doctor’s note or other supporting medical documentation. Now some school boards that oppose the statewide mandate — or that have gotten an earful from parents — are allowing students to go unmasked with just a parent’s signature.