What do I say when people ask where I've been?

I've been gone from school for about a month due to my mental issues. I was put in a mental hospital 3 hours away from my home. I am, well was on the track team. The kids at my school are extremely nosy and pretty much will annoy you until you tell them what they want to know. I really don't want that many people knowing where i've been, but I need to tell them something... any advice?