see, JEE is all about concepts, the knwledge, which the student has to apply in the exam, within a time-limit.... if u think, u can study on ur own, to get thru the topics, thoroughly, then i'll advise, leave the coaching...but if u rnt able to get each and evry topic... then pls continue... as far as doubts are concerned, u can get it solved by ur friends, nd also goiit is always thr..... coaching classes are jst to teach u, to provide u a guidance...they rnot going to write the jee-paper for u...its u and only u who'll be writing the exam... so u only can decide, if coaching classes are actually helping u or not!!!!
as far as managing school and coaching, simultaneously , is thr... dear u have to plan it ....because, an effective planning helps the most...knwledge of the subject, gud time-managemnt, speed, accuracy... if u have all this, then i think no one can stop u frm being an IITian..
gud luck