To me, there's something a bit dismissive about insisting that everyone else's God is actually your God. I think if they say he's different, he's different. Your concept of God is not exclusive, but theirs is. As much as we might wish otherwise, the God they believe in is not the God you believe in - they think you are worshiping someone who does not exist, or something else - but not God.
You are entitled to your beliefs, of course, but so are they. IMO, and this is a pretty abstract concept in my mind - when you say their God is the same one, you're dismissing their strongly held beliefs.
Which, of course, is one reason why the schoolboys think it's wrong to pray to Allah - to them, he's a false God, and they are forbidden to pray to him. I think it's wrong because prayer of any kind should never be forced, nor even lead, in a school setting.