I recently had a coding practical examination in school and I was previously using a Mac book while the exam was done on a Stuck on Windows computer. I'm not a very fast typer to begin with, but silly typo errors every now and then will really make my blood boil especially with the clock ticking. My main concern would be how to get used to the texture of the new keyboard in a short amount of time(like the difference in pressure required to press the keys), other than the fact the keys for both keyboards are very different.
The simplest way is to find out ahead of time what computers will be used for the next practical and either acquire the same brand/model keyboard to attach to your Mac, or find a computer you can use for typing practice that has the same type of keyboard.
That is to say, the only way to get used to a very different keyboard is spend time typing on it -- and the only way to do that, is to have the type keyboard you need to get used to, in order to spend time typing on it.
It's likely you can arrange to use the actual computers used for the practical exam ahead of time -- it should be sufficient to inform the examiners you want to remove keyboard differences as a source of stress and problems on the test, since they're interested in your ability to code, not in your ability to adapt to an unfamiliar keyboard under stress.
The type of keyboard is mostly irrelevant: you get used to a different amount of force needed very quickly, as long as the keys are in the same place. So A-Z, 1-0 and Space you quickly get used to, but there are some annoying differences in the rest because they're not in the same place.
The biggest difference between Mac and Windows keyboards:
- on Windows, the keys on the bottom row are Ctrl, Win, Alt, Space
- on the Mac, they are Ctrl, Fn (if you have a laptop), Option (Alt), Cmd (which serves the function that Ctrl has in Windows), space
The only way to deal with this is to practice.