Yes, it helps people focus. I know people who were able to stay up for long periods of time studying. But its addictive and not good for you. You can get the same results in school if you just stop slacking / prepare. But it does help. Its commonly abused because of its positive effects in terms of grades. I don't recommend it, just study naturally.
Personally, I think Adderall does an amazing job if you are using it for the right reasons. If you are someone who has a problem sitting down and studying because you are easily distracted, then adderall would help you to do that. If you are able to study without being distracted then your grades should increase.
Some people take it to stay up the night before a test and study all night. This would help you stay up for many hours, but it will not help you from being exhausted when you sit for the test and would actually be counter-productive.