Can you write a letter to each member of your local school board and detail these situations? My fear would be that the bad energy at the top of the pyramid at the school would permeate down to the bottom and make it a bad environment. I know you have had trouble with the stepdaughter before, but there must be a reason she was afraid to turn the assignments in, and it seems like you need to make sure a) it's nothing to do with something you and your husband could have helped and then b) that the people whose job it is at school to monitor her assignments are doing so as best they can.
I wonder why they have been so ugly to you in that front office. One would think that they'd be in a better mood what with all the kids gone! It always shakes me up when people act like that and I feel for you!