If I am reading your post correctly, you are trying to figure out if the principal has the authority to take over, not just funds, but the group as well.
If your group has its own 501(c)3 identification number (is registered with the IRS as a separate entity from the school), then the principal can not take over so to speak. You would be considered a separate organization from the school and the school district.
If your group does not have its own 501(c)3 status, then the principal, as head of the school, can step up and decide what happens, not just to the money, but the group as well. The group could be considered more of a school club in this scenario, and the funds would follow suit.
You should ALWAYS require receipts. It is to protect your group as well as keep people honest. It doesn't matter whether the principal got the money from you or from school funds, both would need a receipt for reimbursement. And she knows this.
Look at the group's Mission Statement. What does it say? Look into the Bylaws. What do they say? Have the Treasurer call the IRS and find out if you are a separate 501(c)3 group. If you find that you are, explain to the principal that while you are happy to accomodate her on most levels, that you are a separate group and as such will conduct yourselves as such when things come down to brass tacks. If it turns out that you are not a separate 501(c)3 organization, fix it fast, keep your mouth shut (wow, that sounds harsh...lol), and move on.
It was nice of you to agree to pay for the uniform, however she has funds for that. She should be providing a receipt and thanking you, not giving you grief. Even though you have resigned, you really should follow up to make sure that this gets taken care of.
Good Luck!