I've struggled for the past several years with the parent Whatsapp groups at our primary, and those are only for Reception and Year 2. The amount of messages, a majority of which are unnecessary tidbits of useless updates/notifications, often drown out the occasionally important piece of information. Quite often there are individuals who tend to dominate the messages and the general feeling is of 'groupthink', which results in individual views or concerns being suppressed or held back, out of fear of being seen a different. It rarely feels healthy and I wish there would be a return to the more analog days before social media, where communications from the school were sent home on a piece of paper via the book bags.
The idea of a Whatsapp group setup purely to 'compare' progress of the pupils sounds like a nightmare. From my own experience of these groups I can easily imagine a fair degree of passive/aggressive comments, or worse, outright competitiveness - none of which would help the pupils in question. The problem (again) comes down to nuance - these groups don't tend to allow for it, and everything is thrust into black/white. When dealing with academic progress I think 'sharing' via a Whatsapp group could be counterproductive, and possibly even detrimental, to (some) of the parents. You only need a few competitive parents to be very vocal to dominate discussions, to create an atmosphere where other parents - whose children may be taking a little longer, or whose progress is slower going, to avoid participating.
My own personal view of course, no doubt others may feel differently.