What is needed for a BAH waiver and where can I find example of the memo?

SPC Douglas, I am somewhat familiar with the process as I've been in a similar situation and hope that this may be helpful in some way, the information and process provided is for the USAF and in the case of a PCS, but I would imagine that it is fundamentally similar for the USA (although the length of the school may be a sticking point); that said here is a starting point for a draft letter, it will of course be on official letterhead and will have to be endorsed by the Army equivalent of the "Chief, Air Force Compensation ad Travel Division (Directorate of Force Management Policy). The body of the final approval letter will look something like this (adjust accordingly for an official request letter from you to their office)

-- SUBJECT: Request for Basic Allowance for Housing (BAH) Waiver for 68W Reclassification - SPC Meckey Douglas

-- BODY:  We have reviewed SPC Douglas' request for a waiver to retain BAH at the with-dependent rate based on dependents' address (address here), while attending military occupational specialty retraining.

    Approved BAH rate will be effective for Military Housing Area (MHA) (dependents' address here) on the date member arrived at the new duty station, (duty location here). If dependents change residential address, then member is no longer eligible for BAH waiver and needs to submit an updated waiver request based on dependents' changed circumstances.

    Since this authorization is based on member's request, Family Separation Housing Allowance is not authorized.

-- Signature block

-- First endorsement

Please note that this was approved for a PCS situation, again for schools/training of 20 weeks or less (the TDY en route is likely something to emphasize in your draft letter), it may be a bit more challenging to get approved. Let's see what others on the Army side have to say about current trends, potential templates, and office contact information.  If you have any questions at all, please feel free to reply to this post or send a direct message if you'd rather. I wish you and your family all the best and good luck with your upcoming school!