Request a Functional Behavior Assessment (FBA) in writing.
If the school has not performed an FBA within the last year, request an Admission, Review, and Dismissal (ARD) meeting and specifically ask for an FBA that includes classroom observation in settings where your student is struggling to function. With that information, the ARD committee can develop a Behavior Intervention Plan (BIP), which becomes a part of the IEP. See form 6 on page 37 of the IDEA manual for a sample letter requesting additional testing.
If the school has performed an FBA within the last year, consider requesting an independent FBA. See form 7 on page 38 of the IDEA manual for a sample letter.