Schools provide for opt out for certain types of activities where the photographic access or participation can be controlled. There's no way to control "opt out" of pictures where there is no way to control who is in the pictures. You certainly can't expect the opt outs to be kept off the field/court for the games which are also photo sessions. You don't know the kids so you'd have to depend on picking through them to find any occurrences of the lids who are opted out. Two or three on a team and you may find it impossible to keep them out of the pictures with the children who are interested and whose parents are interested in buying.