Drinks question!

Well, the weather's been warming up lately, and drinking hot tea is going to start to be not so nice soon. While I do make plain iced tea, both green and black, I've been enjoying cold milk tea lately. I usually make it the night before, let it cool in the fridge and then drink it in the next couple days. It'd be nice to make it in advance and be able to store it for, say, a week or so. But for some reason, I think that when you heat the milk up, it'll make it spoil faster. I don't know why I think this, and I'm not sure if it's right or not. If it is, could it be avoided by making the milk tea with milk powder instead?