Setting deadlines for school fundraisers is an interesting topic but something you should think about before starting your fundraiser.
I can not tell you the number of times that a school turns in their fundraising order and then on an almost daily basis calls with additional orders. Although most had probably set deadlines for students to turn in their fundraising orders they do not stick to the deadline.
There are lots of reasons people do not enforce deadlines. The most obvious is that they want their fundraiser to raise as much money as possible so they figure all orders are good orders. They do not take into consideration the extra work involved in their end by continuing to take orders after their deadline passed.
Adhering to a strict deadline is even more important for larger groups since there is a lot of work to be done tallying order forms and cash. Once that is complete and submitted to the fundraising company it is not always simple to add on to an order. That is especially true if you are holding a time-sensitive fundraiser like a cookie dough fundraiser where delivery dates are set up in advance. If you are holding a candle fundraiser or flower bulb fundraiser where firm delivery dates are not required accepting late orders is not as difficult.
The thing to remember about accepting add on late orders is that you might no longer qualify for free shipping if your original order has already shipped.
The best thing to do is to set a firm deadline in writing and include it with the fundraiser paperwork. Do not set the deadline for a Friday or Monday if possible. If you set it on a Friday and students do not turn in their orders they will likely not remember to bring it on a Monday after the weekend. That is the same reason to not set the deadline for a Monday either.
Allow in advance for a grace period before beginning to tally your orders. Set a deadline for a specific date and possibly know you are willing to accept additional orders for 2 more days. After that simply determine that you will no longer accept orders.