If he is on a speech IEP they can also do an OT assessment and then add goals as necessary. It is the philosophy of current education practice to try intervention first to see if the problems can be addressed. Tell them you want an IAT meeting convened so that data can be collected to show that there is a need for further assessment. Also if your son is up for his 3 year reevaluation you are asked to sign a permission form and told which test will be given. State that you want a complete ETR done with ability and achievement assessed using accepted standardized test. This is playing the legal game.
