Helpful information awaits...if you can get it.
First off, this isn't the sort of thing a PTO member should administer. Exit surveys are sensitive. I'm writing about it because I want to encourage you to do some digging:
1. Fact find. Find out if families who leave your school are asked to fill out an exit survey.
2. Utilize. If yes, does your principal have access to the data? Can you know what conclusions can be drawn/what the data shows? (In aggregate, of course, as you shouldn't see individual responses.) You can then use this additional data to help shape your goals for marketing, both recruitment and retention.
3. Encourage. If no, of course there's no data for you to utilize, but push your school and/or district to implement one. It's helpful information you are missing out on!
Now, it's true that not every departing family will tell you why they are leaving, and indeed, those unhappiest may be more inclined to respond than others, but I think when it comes to data, the more the merrier. Have you already determined your school's weak spots? And are you utilizing your school lobby?