Make a comment on the FB photos saying it would be nice to see how the 'rest of the children' are enjoying the residential as it seems only xx number of kids are on this trip!
I am a teacher and I like to think that I am fair when it comes to 'choosing children' to be representatives of the school or for photo shoots. When it comes to performances, it can make it tricky to be 'fair'. Being fair isn't about treating everyone the same; but giving everyone the opportunities they NEED to succeed, excel and shine. We all know that many PP children need more intervention at school to allow them to achieve and access the curriculum the same as their 'more affluent' peers - but that's another story.
However, I am also a parent with a FS2 child in a large village primary school. I like where I live but when DS started school, it opened my eyes to how much favouritism strikes the children according to 'who your parents are'.
In my DS's school, the majority of the parents were born and bred in the village, all grew up together, went to the same school (the village primary), married each other and bred! It is noticeable in the school as many of the teachers have been there since the dinosaurs reigned and remember the kids they taught and now teach 'their' kids! The children who are chosen to eat on a special table at the end of the week are the same ones - parents who are actively engaged in the PPA, parents who volunteer in school, parents whose older kids have been through and therefore have had a 'link' with the school for 20 years (exaggeration - but not if you include the parent's own education). Even the children who win the school competitions and are chosen for the sports team! It is ridiculous and then some parents whinge and complain when their little 'Tarquin' isn't chosen, or has been 'made' to take part in a class production and even complained when he/she has been told off because 'they don't want to put the pens away' 
I tend to drop off my child and pick him up (on my days off) without engaging much in the playground. The gossip, the politics is too much to bear for the 10 minutes that I am there for! One lady I know was BULLIED by parents over FB because she happened to make a comment about how something was arranged and all merry hell broke loose! And those same parents were also close friends of members of staff (believe me when I say it is the school business manager and secretaries who run the school!), PPA members, school volunteers and the same ones who park right up onto the grass verge outside of school 20 minutes before school starts/finishes to ensure their 'Tarquin' doesn't have to walk far and of course ex-students! I was ready to move DS before he even started due to the clique-iness, politics and 'pecking order' of the playground mums.
I keep my head down now. We don't do play dates either because of the gossip of 'whose house is better' tales!
Totally digressed with all that!