Hi....I don't know if I have lots of advice, but feel I should post a reply for you !
I suppose you need to consider how awful the other school would be if your boys didn't pass and that was the only option.Do you know any families whose children attend ? Anything positive about the school ? Do you know what levels your boys are currently at....do you think they might be grammar material ? What age are they ? Extra work one to one makes such a difference and it doesn't have to be a traumatic, stressful affair if handled the right way. How competitive is the Grammar to get into ?
How easy will it be to find a house, sell yours etc and move into the other catchment area ? I don't know the area, but I'm sure someone will help or maybe have a look in the specific part of the forum for your area ?
Lots of questions...I have my fingers crossed for you that you will find the answers !