Wow, thanks for all the replies!
Firstly - yup, it's definitely a decision for his parents and I'm glad I don't have to make it! However, I'm a partner of one of those parents, so I help DP think about those decisions. This is just me, finding out a bit more about the situation so I can be helpful and informed. I also care about DSS and want him to be happy!
It sounds like the resident parent does get the final say. So that's useful to know. We're in the middle of trying to do mediation/communicate via solicitors, and DP is hopefully going to have the opportunity to discuss this with ExP during that.
DSS is only year 4 at the moment, so we're just doing some thinking now so we have time to discuss it. We (DP, me and DSCs) had a chat about it today, as the topic of secondary school came up, but DP kept it very light and was very balanced - his brother got a scholarship to boarding school and went away, so it was easy to have a conversation comparing that to our experiences at the local comp. DSS was very keen on being able to see us during the week, but that's just off the basis of one conversation.
Obviously the best bet is DP being able to have a balanced chat with ExP and come to the decision that is best for both DSSs, but which has fairly taken into account their education and their ability to maintain a relationship with both parents. However, we have been categorically told on many occasions that DP is not important in the children's lives, so I don't hold much hope out for that. This is also something else to consider - we currently struggle to see the boys regularly, as ExP regularly changes contact at short notice. I suppose this will change as children get older and make more independent decisions.
I don't know what the schools are like in the halfway town, that's something to research. However, I do think you have to consider more than just the quality of school. As balia says, if he goes to school further away from us, then his social life etc will all be based further away from us. We have the opportunity for this not to happen, so it would be good if it could be considered. For me, it feels like the children's relationship with their father is equally as if not more important than a good school, and it would be very sad to harm one at the expense of the other. Schooling is obviously a very emotive topic, so this won't be a straightforward one to resolve.