do you know for sure why they have left?
playground gossip can be wildly wildly wrong (I am a governor, so I often know the real story and what goes round the playground is mins boggling at times.)
eg at the moment a lovely long term member of staff is leaving to a deputy head's job. The playground is saying she's going because she applied for deputy last year at our place and didn't get it and the school made a mistake grumble grumble. Thing is, she didn't apply for the deputy at our school!
So, do you know from a member of staff what the issues are? Do you know anyone at the other schools they run?
I think I would move to be honest, but that is a hard call to make.
You do have a new head. You could ask to see them, and be honest - 1/4 of your staff are leaving, the rumour is that they are unhappy about xyz. What can you say to reassure me as a parent.
hear him/her out and see what you think.
In things like this, our tendency is to keep with the status quo. In reality this usually means we move too late. (see comments on thread sabout moving kids due to bullying, everyone always says they wish they had done it sooner) I would rather move a kid at beginning of year 8 than year 9, even though it means new friends etc.
One of ds friends is at a school that began a troubled few years as he was in year 7. They are now year 10, none of his needs have been addressed and he has effectively lost a couple of years of school, add on a nasty incident which I doubt would have happened in a well run school. School is now sorting itself out, but it feels like that boy has felt the full effects to his detriment.
Oh, and you will not have permanent replacements in most of those roles in September I think. (might be wrong but I think it is unlikely) so he will be getting cover teachers.