Just do it. If they are showing you how to do it before hand, it can be done. And remember, if it is difficult for you, it is difficult for the majority of people. If this chalk carving test is the only thing keeping you from going to this school (assuming that you want to), then do what I did and practice like hell till your motor skills are relying more on muscle memory and you start seeing this carving in your dreams. This is what my mentor told me and I went to work with it. If you want it bad enough, you will get it done.
BTW: if you can get the same material for practice, look at how the material reacts with moisture. The chalk that I had was much more easy to work with when slightly wet. (Use spit- I know it's gross) Come to think of it, I was licking the hell out of the chalk figure being pressed for time- LOL. Good Luck!