Hello Aspirant,

11th and 12th class would be Senior Secondary, and Senior Secondary School Teachers are known as PGTs (Post-Graduate Teachers) . So in order to become a PGT , you need a Post-Graduation degree in the teaching subject of your interest ,which in your case is would be chemistry (non-professional, degree courses) and a Bachelor of Education (B.Ed.) degree. So after 12th , you should take up B.Sc Chemistry, then M.Sc Chemistry followed by B.Ed.
If you want to teach in a government-owned school then you must clear the Central Teacher Eligibility Test (CTET) exam.Cracking CTET will help you become a teacher in schools under the Central Government (e.g: Kendriya Vidyalaya). You can also appear for STET (State Teacher Eligibility Test) cracking which will enable you to become a teacher in schools under State Governments. In private schools you may not need such entrance qualifications. Some private schools may not demand B.Ed as well , a PG in the subject may do.

All the best