You will probably need to get with an attorney to verify that this is correct and, more importantly, how to go about enforcing it, but if you have joint
legal custody, then you have the right to obtain school, medical issues, etc.
In all probability, you'll start with a
certified letter to the school, doctor, insurance company, etc asking for copies of the records. You may or may not need to enclose a copy of your divorce decree showing that you have
joint custody. If that doesn't work, then send it to higher level people. School Superintendants are generally more aware of the legal implications than, say, a teacher and may be willing to give you what you want even if the teacher won't. In the case of the doctor, send it to the owner of the medical practice (asuming it's not the doctor). If THAT fails, you may need a letter from an attorney to force the issue. If even that fails, you'll probably have to ask the court for an order.
This is one of those things where you have a legal right to that information, but it may easily take a great deal of work to enforce your rights.
Note, however, that all bets are off if there are any allegations of abuse since they don't have to give you info if you're accused of any type of abuse. if that's the case, you definitely need to see an attorney to get anything done.