In IISER, for applying through SCB channel, only Physics and Chemistry are mandatory subjects. Since the IAT aptitude test conducts the exam with Physics, Chemistry, Biology and Maths with equal weightage, you need to study Mathematics too, whether or not you had Maths in class 12.

No, it is not mandatory to have maths as one of your optional subjects at plus two level for BS-MS degree programmes. However, for admission to BS degree programmes in Economics and Engineering  sciences at IISER Bhopal, Mathematics as one of the subjects at +2 level is mandatory.