Got on Ritalin for my half yearly's and it was like me and that test paper were the only thing in the universe. Wrote like 6 pages for a 45 Min essay and got above 90, probably would have done just as well without it but it keeps you concentrated like nothing else.
Wouldnt recommend anything other than things that aid concentration ands defs no shabs
also probably wouldnt do it for the hsc, studying will help more