That's a difficult situation, but legally those files belong to the group and not to her. Have you gotten the principal involved? Sometimes having the principal as an authority figure conveying your message can help. It's good that you've been able to add yourself to the account. You should also make sure that she has been taken off the account. Ask the bank for copies of statements going back a year or two -- typically if something fishy is going on you'll be able to tell pretty quickly.
Unfortunately, your situation isn't that uncommon -- lots of groups run into trouble getting the old records. In most cases, there's nothing bad going on -- maybe the treasurer has left the records in a mess and is telling herself she'll update them before turning them over, for example. Is there anyone on the former board who is still involved? A visit to her house by someone she knows might get the job done. Good luck!
- Craig
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