I use to send out a letter to the classes I was going to take pictiures of and I only had 1 come back a refusal.
If you tell the parents why your taking pictures and where the photos will be and there will be NO personal info about the children you will not have any problems.
Make a copy of the signed form and give it to the office.
Even posting the pictures on the web is ok if you follow the No person info rule. Take the picture down after a couple weeks.
I had all my 5th graders "adopt" a rancher my last 2 years and I had their pictures up and posted on a ranchers site about what the kids were doing and what they had learned. The ranchers sent letter and pcitures and emails to the kids about what the cows were doing. The parents enjoyed being able to see thier children learning from other adults.