and we haven't found a way around it. Is there any way for a student who has filled out a form on a Mac to save the form while PRESERVING the extended rights?
There's an easy way around it. Make sure they are using Adobe Reader to fill in the form. 99% of these cases are because they are using the Mac Preview application to open PDF's and it will corrupt Adobe forms.
Since it is a competition, just let the students know that they will lose the competition if they fill out the form in Preview instead of Reader. After a year, their teachers will make sure the students do it correctly.