Is your PTO's money in a bank account under the school's tax id number? If so, I think they could conceivably try to take it from you since you've in essence already given it to them. Vote to reallocate soon, and then consider opening an independent bank account with your own PTO tax id number if you've been using the school's.
I agree with JHB. If you are an established PTO and you already budgeted funds for field trips, but the board of education will not allow them, you can reallocate those funds.
Maybe think of having a day-long event at the school where the kids can let loose a bit in place of the class trip. You will still have the per/child amount to use, but for something at the school.
If you are an independent entity - and ideally established as a 501(c)(3) organization - they should have no rights at all to your funds.
However, if the school participated with you in fundraisers with the explicit goal that the funds be used for fieldtrips, I can see where they might want a voice in the plans for those funds.
If it were me, I'd reallocate those funds - hopefully with the cooperation of your principal. To me, an obvious avenue would be that if the kids can't go out to field trips, could the funds be used to bring programs in or to help put on some fun or educational in-school event. The kids would still benefit from a planned group activity.
Then again - maybe equipment or supplies is what your school neds most.
The outcome also depends on the amount of funds and the percentage of your budget it represents.
Example 1: The PTO raises tens of thousands of dollars for playground equipment and that project gets cancelled. f I were a parent (or administrator) who supported those fundraising efforts, I'd want to know how those funds would be used and who would have input.
Example 2: The PTO had multiple plans, a $25,000 net budget, and a particular program budgeted at $750 gets cancelled. As a stakeholder, I probably wouldn't be too concerned as long as the funds were used to support the PTO mission.
Bottom line - I think it would be unlikely that your district would try to attach the funds. But I would start planning an alternate use for them and I would solict cooperation from the principal and input from your stakeholders.
I am just learning of a directive that was passed days ago from our superintendant: effective Nov. 1st there will be no field trips approved for all grade levels (an exception has been made for outgoing students i.e. 5th - 8th - 12th). My question at this point is does the school dept., building principal or superintendant have any rights to our cash funding that was slated for these trips? We have it written into our bylaws that we are not responsible for needs that should be provided by any of the above. The purpose of cancelling is lack of funding and overuse of the substitute budget. Many are worried that they will try to attach our funds to suppliment their own. Can this be done? I would think not.