I have completed my 12th today...and i want to know that droping 1 year for entrance exam is good idea or bad?

If you are really passionate about the course,go for it.Just one suggestion,join a coaching class,dont do it from home.Trust me on this,do join a coaching and not simply test series.

if you want to drop one year then this is best only in one case you have prepare exam for this year and get good result in entrance test if you can get admission IITS level colleges

Bad idea. Take a course of your choice and devote some time on the subjects you like, still you will have ample time for preparing for entrance exams. Just preparing for entrance may bring monotony or even laziness.

It is a very good idea , if you really don't waste your time and prepare for the Entrance.

It could be a better idea

Well it all depends upon your preparation. Dropping a year for whatever reasons is not advisable, but doesnt mean it cant be done. If u think tht an extra year will help u prepare better and get u tht admission u hoping for then its ok, otherwise do wuteva u can to save tht xtra year. best of luck!