There are 2 things to this. Let's say if you give your Board exams and then you are short of few marks for being considered "PASS" in any subject, then yes Grace Marks are allotted. At times it is also done during the process of revaluation. But now that you have written Mathematics as an isolated or an additional subject, it totally depends on the decision of the invigilator. He/she might add grace marks or might not. So we cannot answer this with certainty. The call is taken on a case to case basis.