I just filled out my Need Access forms with my parents last night. It would be just about impossible for you to estimate all the crud they want you to put down (i.e. medical expenses, interest earned, tax deductions taken, etc.).
If your Dad doesn't want YOU to know about their financial situation, he can fill out the application by himself with his own password and indicate on the form (there is a box) that he doesn't want anyone talking to you about his money situation.
If your Dad doesn't want THEM to know about his financial situation, I dunno about that, but I would call each school and ask them individually how you should deal with it.
Do you have a mom that would be willing to sneak you a copy of the taxes?