I would suggest you to complete your 12th followed by Undergraduate or Postgraduate Education in some of the Top Technical Institutions in field of Aerospace, Avionics and Electronics like IIST Trivandrum, Top IITs, IISc Bangalore and so on.

However, since you insist on the Polytechnique Route. I can give you a solution. After your Diploma, you must go for a AMIE Associate Degree from Institution of Engineers or B.Tech Lateral Degree in either Mechanical, Electronics or Aerospace, followed by M.Tech from Top IITs or IISc Bangalore. ISRO has more vacancies in domains of Electronics and Mechanical than Aerospace, hence you must keep the latter as your last preference.

Following a M.Tech or PhD, there are many vacanies annually at Different Centers of ISRO under Scientist C or D, and you can easily secure an Opportunity by aceing the Qualifying Test conducted by ISRO followed by Interview.

However, after your Polytechnic degree or Diploma, you still have certain Opportunities with ISRO at Group C and D, Technical Assitant or Apprentice levels, but they are scarce and you must invest on higher education.