The school district should have given you Procedural Safeguards. The doctor should contain info about the steps to take when parents and sd do not agree.
When there is a disagreement, parent or sd can request the IEP meeting be reconvened. During this time, each can gather additional info to support their opinions.
If the parents and sd cannot agree upon reconvening the meeting, mediation is an option. In most states it is "voluntary," e.g., one or the other party can refuse. When this happens, it goes to due process IF one party files due process. There is also a Complaint procedure.