If you want to do timed practice or something, then it's probably a good idea to use short words like hi rather than long words like hello. It probably only saves a fraction of a second, but if it's not English you're learning, using few and short words in that language probably isn't gonna hurt the learning.
Not that I use timed practice when I know I have weak skills, then I practice those normally first.
Edit: "Hi" is now accepted.
Or you can learn fast typing, and in this case "hello" will take you nearly as long to type as "hi". Timed practice needs shouldn't be something to consider, as it could lead to use improper abbreviations for the sake of the watch, instead of the sake of the grammar correctness. It's dangerous. They cannot accept shorter alternatives only because it's shorter, they have to ignore the length.
I disagree with "it's not English you learn here" part.
If they chose to translate "hello" with "halo", it's probably because a more informal word for "halo" does exist, that can be translated with "hi".
For instance, in the French course, Hello/good morning = bonjour. And Hi = salut.
You can use "hello" for "salut", but can't use "salut" to translate "good morning", the reason is that there's a difference in the formality.
Probably the same here, there's a difference in formality between "hello" and "hi", they are not the same in English, maybe there's an informal "halo" in Indonesian? I'd be happy to know it.
So, what is the very informal greeting in Indonesian?