I'm not disputing that there may be circumstances that a Whatsapp group may have info that could be requested by FOI. I'm disputing that this situation amounts to that on the evidence we have.
So let's tick off the elements needed.
1. Is a state school in scope for the FOI - yes, tick.
2. Is communication between employees of a organisation in scope for FOI likely to be in scope for a FOI request - yes, tick.(but see 4 below).
3. Is communication via private WhatsApp messages in scope for FOI - yes, tick.
4. Would discussion on the behaviour of a pupil in the classroom between teachers at that school be considered information pertaining to the business of that organisation - yes, tick.
The only one that is even slightly in doubt would be 4, but it would be hard to argue that if a parent made a FOI request for the release of information containing discussion of behaviour of their child while in the classroom between professionals at that school that this would not be considered to be in scope.