Hello 

People do it along with class 11,12 just to utilise their time so that they do not have to take a year gap after completing class 12 and to practise more.Honestly their class 12th studies are not given much importance because their major focus is on the JEE preperations.If you think you can manage both together then yo can do it with class 11,12 and if you want to focus on class 12 then you can do it after class 12th.

So this is a personal choice whether you want to do it with class 11,12 or after class 12.