One of the first things that I look for is an interest in the subject. I don't expect them to know everything, but a bit more than just the basics. If they are interested, then I've found that they will do well in their training and their exams. You will never have much luck with people who do things because they have to. You need people who do things because they want to.
As for kids straight from school, I always want to know what other activities they do. Anyone who belongs to a youth organisation like the Air / Army cadets, scouts etc would always get a second look because they have the get up and go that most kids don't have, plus they are used to taking orders and have some discipline in their lives. Also, believe it or not, anyone who has worked at MacDonalds etc would also get a second look because they are used to working hard and again, will be used to taking orders (and I don't mean from customers...).
I also want to ensure that they will last to the end of the course. What are their long term plans? If you can find people who have an idea what they want to do they are more likely to survive. I know that Kids don't always know but places are valuable, so you can afford to be picky and take the best of what's on offer.