hi,

see the 11th and 12th are the two years course and with out gathering the proper knowledge of 11th you can not study 12th course. it is necessary to complete the 11th course and then take admission to 12th.

however if you go for national institute of open schooling i.e NIOS you may pursue senior secondary course in one year, which syllabus is 12th standard. you need to have the certification of 10th standard pass from a recognized education board. NIOS many branches and you need to contact with the nios branch nearest you home.

follow the below link to know in details:

https://www.nios.ac.in/departmentsunits/academic/senior-secondary-course-equivalent-to-class-xii.aspx