It's Not Easy Being Green - S5-E16
Question: How could Bobby and the rest of Mr. McKay's class have showed up at Strickland Propane without Hank knowing? Surely the students would not have been allowed to go on a field trip without a parent's permission. If Peggy had signed a form at some point, she mostly likely would have mentioned to Hank that Bobby would be at his workplace.
Answer: Peggy is a substitute teacher. She may have been teaching at the school the day the permission slips were sent out and Bobby could have just walked to her classroom, had her sign it, and taken it back to his teacher and both just forgot to tell Hank when they got home. ("There are ways" is my point).
Captain Defenestrator