Well, you need to do research first. Times have changed for better.

Hello
Yes you can join .
But if you are really dedicated to get admission in IIT ;NIT than you should join in 11 the class because if you join in 12 th class it will be somewhat difficult for you to match or to understand some of the concepts at that time which they have told in 11th.
And you have the study 11th also ( for JEE ) and practice more.
And you will have 2 years for the preparation for JEE .
So take this decision very wisely and after thinking.
Good luck

Dear student,

Am not promoting any institution over here but Yes if you want and think there is a need of some professional classes then you can surely opt for the admissions in Allen coaching classes,it may give an extra support and help which could be beneficial for 12th Intermediate board examinations..