I think your plan sounds fine. Sure, tell the teachers what you've got planned -- and encourage them to come to the meeting to make sure the reimbursements are approved (maybe they can talk a little about why it's important, what they bought, etc.).
As long as the expenses are for next year (ie, the year from whose budget you are taking the money), I don't see why receipts from July would be a problem.
I am the President of our PTO next year. This school only has one grade of children in it so the PTO and Board basically change every year. We plan to budget for start up money for our teachers to use at the beginning of the year. They will need to submit a receipt and we will reimburse. I believe our fiscal year starts in August. (I have seen no Bylaws yet and we are not 501c3)
My questions are:
I would not plan on giving out those reimbursements until after our first PTO meeting of the year so that the members can vote and approve the budget -thus approving that expenditure. Does that sound normal?
Would it be OK to let the teachers know in advance what we have budgeted so they can plan - meaning we are assuming it will be approved?
Then, I know the teachers will do much of their shopping before the school year starts. I am OK with them holding onto their receipts and after the budget is approved turning them into us to get reimbursed. Will we run into problems if someone went shopping in July (previous fiscal year) and we reimburse them?