The subjects chosen at the B.Ed stage need not necessarily be the same as that of graduation stage. Normally candidates prefer to opt the subjects with which they have acquired their graduation degree. However, the same may not be mandatory and the selection of the subjects at B.Ed stage would also depend upon the candidate's interest and passion.
So while including Mathematics as one of papers at B.Ed stage, you should be familiar with the cirriculam of Mathematics taught at the graduation - level and see to what extent, you are comfortable in grasping this subject. You can even go online correspondance course in Mathematics dealing with Algebra, Trigonometry, Differential and Integral calculus, Differential - equation, Vector - algebra etc so that while teaching Mathematics at the school - level, you are not loosing your confidence.
The option lies in taking up a post - graduation degree either in Social - science or Mathematics depending upon your interest after completing your B.Ed course. At the later stage, you can write NET so that you may be inducted for the post of lecturer in a constituent college after going through the selection procedure of NET.