While it would have been the decent thing to do to put you on the list, if you've got a court order showing that you're a legal guardian, it should be a simple matter of providing that information to the school and being granted the access you desire.
If you plan to bring a case to modify custody, you'll need SOME form of documentation that not only is there an alcohol problem, but that the problem is having some impact on the child to the degree that the court would reconsider primary custody status. Custody changes aren't often based on what's changing in the parent's life, but what's changing in the CHILD'S life. If dad's issue isn't having some noticeable impact on the children, then it's not likely to sway a change in custody. Same goes for dad's threat to you - HE doesn't get to determine if you get to see them or not. That's why we have judges and court orders. If he wants to ask the court to terminate your parental rights, he's going to have to prove to the court's satisfaction that you have created such a negative impact on the children as to merit that extreme step, or, show the court that you've abandoned the child for a substantial period of time (which, if you're working towards getting more involved in checking out his school status makes that arguement on dad's part a moot point and a whole lot of hot air).

