That's a sad start to the school year for your family Ashcombe, especially when things need to be kept upbeat and positive.
There could be so many possibilities as to the reason behind it.
Firstly, has your daughter spoken with the other 3 parents to get the full picture? This isn't being sneaky. Parents discuss school life constantly in messages, it's part of today's world, and they often pick up truths about what was actually said or happened. Before seeing the Head it is good to have the proper picture from all involved.
I also feel * Lucca* may have a point. Was the page removed because it was at the beginning of the book? Was it also removed straight after the title or further along the line? No one wants to look back at a glaring mistake every time they open the book and pages are sometimes torn out to avoid this, especially before there is any accompanying work. It would have been far more upsetting if it were after a lesson of exercises. Of course it should have been done in a helpful manner and not aggressively.
I would have thought any heavy handed behaviour from a teacher would immediately get back to the head anyway. Your daughter should be in a very good position as a Parent Governor and a deputy Head herself to gauge the atmosphere and communication within the school. She is well equipped to act appropriately via the correct channels.
Finally, the concern should be for your little granddaughter. If this incident has knocked her confidence badly and she is still distressed, then she too needs an explanation. If, however, she now appears unperturbed, it us important to let her move on from it quickly. Children themselves have a very strong sense of fair play and justice.