You have to achieve the aggregate cut off.. and if you have failed to achieve a cut off in one of the individual sections, only then you can be offered a preparatory course.. that too a list of the highest scorers will be published... So marks needed will be close to the aggregate cut off for SC/ST.
I hope this solves your query...
Post any further doubts.. We are happy to help.
Wish you a very good luck... :) :)