Colleges in Kolkata

Mathematician - You can choose to pursue research (become a scientist) in your area of interest in maths and if you choose research, you can go into the teaching sector also. Check out institutes like TIFR-Mumbai, IMSc-Chennai, Harish Chandra Research Institute-Allahabad.
Chartered Accountancy - If you like auditing, taxes and accounting, you can think of preparing for CA too.
Banking - Banking is another sector that you can get into with mathematics.
Actuarial Sciences - If you love data and maths, actuarial sciences would be a great field for it.
Data Analysis/Operations Analysis/Project Development - If you want to work in the private sector, you can look at these profiles.
Military Operations - Mathematics degree would also allow you to work in the military sector, scientists are always required to work with military and development (DRDO, ISRO, NAL, SETS, etc.). You can also join the navy as a Commissioned Officer in their education or logistics branch.
Market Researcher - There are a lot of companies that you can work with to identify market patterns and trends, and help in analyzing these trends.

Yes you can apply for b.ed by fulfilling the following eligibility:
B.Ed. Eligibility
The minimum qualification required for entry into B.Ed. course is Bachelor of Arts (B.A),Bachelor of Science (B.Sc.) or Bachelor of Commerce (B.Com.) form a recognised board/university with at least 50% marks.
Students required to appear in an entrance test conducted by the various states and Independent educational bodies and universities for admission in various teaching institutions attached with it according to the rank of the candidate in the entrance exam.
These exams are generally held in the month of June-July and consist of Objective type questions about English, General Knowledge, Aptitude, Basic Arithmetic Teaching Potential and one local language in some states. The results of these exams are generally out by July/Aug. Candidates go through practical training courses, apart from theory classes.