B.Sc Maths

No. The answer is obviously a no. You'll have to start from the very starting.

Did you mean you want to pursue Maths (Hons) in post graduation? If yes, then you will have to appear for entrance test of every government institute you want to take admission in. If you want to go in IIT you will have to appear for IIT JAM. If you want to go fir research in your subject along with post graduation you can apply for TIFR. For private colleges, there arena't any entrance tests.

Yes, of course, you can get admission in maths hons in Delhi University at least and other state Universities too. There may a little deduction in your percentage though while calculating for best 4.

If you are really determined or really want to take admission in renowned government institute like Delhi University, you can drop a year. But that according to me will be wastage of time. You can take admission in state University. They will surely provide you admission. State Universities like MDU or KU will definitely provide you admission. In case you go for dropping a year, you can also prepare for IIT-JEE.

You can go for M.Sc. in any of the above subjects. For that, you will have to take entrance test. Institutions like IITs provide admissions on the basis of IIT JAM result. Option for civil services is always there. You can take SSC CGL exams. If you have good teaching skills, you can apply for coaching institutes like ALLEN or Akash.

Almost every institute in India gives admissions for graduation on the basis of 12th marks. So, do well in your boards. If you want to go for research, you can take admission in IISC, Bangalore. Admissions there are given on the basis of IIT-JEE result.

B.Sc Mathematics graduates have both, government and private job opportunities available in front of them. Some of the prime recruiters are:
Schools
Colleges
Coaching Centers
Banks
Government Organizations
Financial Institutes
IT Firms
Research Firms
Consultancies
In the above mentioned work setups, B.Sc Mathematics graduates may take up the following job profiles:
Teacher
Lecturer
Instructor
Research Professional
Consultant
Banker
Loan Officer
Mathematician
Ample self-employment opportunities are also available in front of B.Sc. Mathematics graduates. They may start their own tuition or coaching center. Regards.

B.Sc Honors is designed with detail course. But if you want a more balanced and multi-disciplinary approach, it is better to pursue a B.Sc. General course for under-graduate studies. So one whose interest is in studying in details (here in case of mathematics) go for B.Sc. Honors. In B.Sc. Honors makes your foundation more strong especially in mathematics. But if one wants to be balance in all respective subject then he or she goes for B.Sc. General. For any further query, please feel free to ask.

Please check for your eligibility to apply for the JECA exam from the criteria mentioned at the link provided below:
https://www.shiksha.com/it-software/exams/wbjee-jeca
WBJEE JECA 2018 Summary:
JECA or Joint Entrance Examination for admissions to Master of Computer Applications MCA is conducted by West Bengal Joint Entrance Examination Board (WBJEEM). The exam is a gateway for admissions to MCA courses offered by colleges in the state of West Bengal. Regards.