The North Penn School District is appealing the tax assessments of 26 commercial and industrial properties in Towamencin, Hatfield, Upper Gwynedd and Montgomery Townships.   The school board took action to approve filing the appeals at a special meeting Wednesday night.   District officials say some significant dollars are at stake.   The properties targeted for assessment appeals include the Penn Brook Gardens Apartment complex on Church Road in Upper Gwynedd.   Seven properties owned by Philadelphia Suburban Development Corporation in Towamencin are also on the list.