Don't worry about blood tests any more. The last one I had to do was just a tiny finger poke and they take the tiniest amount of blood into this plastic card thing.
I had a teacher with the same as you, one leg shorter than the other. He actually had one of his shoe soles made a little thicker than the other. I always wondered about that, but he didn't seem to walk with a limp that way.
You could do shoe inserts or have certain shoes re-soled to accomodate the difference. It depends on the comfort of your feet in the particular shoe.
They most likely will do a physical inspection of your spine, the curvature and stuff like that, nothing invasive. But do take care of your joints too, you will want them to work properly when you get older, lol.