Hi,

IIT does not offer 10+1 or 11th and 10+2 or 12th class courses. They provide graduate ,postgraduate and P.hd courses.

At IITs you can join undergraduate courses after 12th through JEE exam.

You can join post graduate programs after qualifying exams like GATE etc.

If you want to join IITs you can take admission there after 12th provided you should score minimum 75% of Marks if you belong to general category. Then you have to qualify JEE main and JEE advanced.

the more information about jee  exam you can get from the article - https://engineering.careers360.com/exams/jee-main

Hope this helps.