At my child's school, photos were handled by a vendor that the PTA arranged and managed. Parents had option of ordering directly with the vendor by sending check and order form to the school, or by ordering pictures via the vendor's website.
If you don't know how school pictures are handled at your child's school, I suggest contacting the school office first to learn more about it. Here's a directory of U.S. schools' contact info, in case you don't already have the school's phone number handy: http://www.education.com/schoolfinder (Please note though: you can't email the school via SchoolFinder.)
In case you're unsuccessful in tracking down a copy of the photos taken at school, below is an activity you and your child can do at home with friends to capture a school photo for this academic year. Alternatively, you can contact local photographers in your area.