Kindly note that most State Boards or the board that enabled you to do the 12th examination itself, will allow you to complete the 12th standard. Also, please do note that some Universities allow you to directly register for a bachelor's degree in any subject through distance education, if you are above the age of 25. However, please do note, many organizations, and even some schools, do not recognize such degrees.
So, the best method for you to get ahead in life, is to appear and get a pass in any of the State Board examinations or the Open School. You can get further details from any good coaching institute, which are there in any nook and corner.
You have mentioned that you already have a good salary. Nevertheless, please do note, passing the 12th standard State Board examination is a must. Follow that with a distance education BA and then the B.Ed from the Indira Gandhi National Open University, which will help you to go ahead in life. You will also get a promotion in your own school, or you can switch to another school.
Since you have not mentioned your subject of interest, choose the best and then study both for the degree and then the bachelor's degree in education. You will soon find that you that you have a superb status.
Please do not go in for short cuts. There are hundreds of advertisements where they say, get a bachelor's degree in one sitting. Many boards of secondary education and the CBSE will not approve your qualification. Please do note, there are audits for this purpose and nowadays, everything is up loaded on to websites.
Go for the long haul, as teaching has no age limits. I know teachers who are superb teachers of English or Mathematics or even Social Science, even at the age of 76. They take private tuition, at this age. Remember, sky is the limit, if you are talented.