sir tell me upto what level i should prepare in 11 th class for iitas it is becoming difficult for me to manage my school with iit.(i have joined askiitians )

see, iit syllabus is same as ur school syllabus . The only thing is that you have to learn the concepts in detail and do keep practice of more and more questions.
Its very easy for you to practice more questions you have the full time you can easily handle it.