If your account is private, both Facebook and Instagram ( and completely private with no posts viewable and closed friends list, and minimal biography information) there is nothing wrong with sharing the back to school shots.
BUT.
When a parent has a public Facebook or Instagram and posts an image of a back to school shot, location tagged, with the school name, and a picture of their child - they not only often name the child in full, but their own accounts are open- so further details about the family and the child are revealed.
The days of a newspaper safely publishing first day kids should be gone, because a child's full name may be cross referenced with parent surnames if a school has a Facebook or Instagram account, and said parents interact with it.
Aside from the risk of online grooming (additionally, a surprising number of parents also link to children's social media if they are slightly older) the data provided within these public accounts often contains enough information to construct a false identity.
You can gain a child's DOB, full name, parents names, jobs, educational institutions, and DOBs, and often street addresses, plus where a child was born. By the time a child reaches the age where they wish to gain bank accounts of their own, and ID documents - they can find their identity has been stolen.
Secondly, what these families with no account privacy also hand out are enough details about the family for a stranger to convince a child they are indeed a family friend. Name of the pets, family nicknames, where the child has been on holiday, mum's best friends, all the relatives names, what the child did for their most recent birthday and the names of their siblings. Enough to convince a child that this person does indeed know their family.
Finally, little kids in uniform, and little kids at dance classes posted on a proud parents public Instagram account are paedophile fodder. They aren't going to be leaping the fence of your home, but they will collect the images of children they find attractive, and add them to their libraries which are shared amongst others of their ilk. Open parent accounts then provide many more image of said cute child and thus many more images are copied and fielded around the world to be found by police when searching for missing and trafficked children amongst these copied images from Instagram and Facebook. Even the idea of an image of my child being a mastubatory tool for creeps worldwide is enough for me to never post anything about my kids, even if I do have very private social media. ( Back to school shots are a doorway to locating that cute kid initially, as is searching school location tags).
Personally I think, spamming your child across the world via social media shows no respect for their privacy, and their future digital footprint. There are risks involved. Not securing your Facebook account, or your Instagram is incredibly silly and irresponsible if you wish to share images of your kids.
Long ( Sorry).