Many people don't know, there is a statute in Maryland (and many other states as well) which protects a parent's right to medical and educational information, regardless of which parent has custody. In certain circumstances, the court may restrict that right however. In addition, if parents do actually have legal custody, they will need to share information between themselves as well.
Regardless, sometimes my client have difficulty in getting the doctor or school to abide by this statute, in which case I send a letter with a courtesy copy of the statute. If there's litigation pending, a subpoena may do the trick as well.
For more information, see Maryland Code Annotated, Family Law Article 9-104.