Good luck!! As mentioned to get into zookeeping courses, you have to already work in a zoo. If you live near a zoo, definately volunteer your time and if you're very lucky, the right position will come up while your volunteering. Its a very hard industry to get into. Ive done work experience at 3 different zoos, completed my vet nursing cert at TAFE and still couldnt get a position. (I was told on the quiet that a recently married, 29yo had buckleys chance of getting a job as I was at 'childbearing' age). I think if I had lived closer to a zoo and could have volunteered time on a regular basis, I would have gotten a job eventually, unfortunately I live hours from any zoos (I mean real zoos, rusa park is about an hour away) and could only go in 2 week blocks. You're only young, all I can suggest is do a science degree once you finish school, this will put you above everyone else to start with.
Good luck, its a great job!
Cheers