Yes ! you can absolutely score 90% if not more. I will tell you my story. I was a nda enthuthiast , I just prepared maths . Physics and Chemistry seemed a distant relative. I went for ssb in january, got selected , spent the whole month in post ssb obligations, went for medicals in feb .. all this time I had no clue what the syllabus of chemistry and physics was.. :p my boards were from 1st march and on 28 feb I was in train travelling from delhi to pune after medicals.
Still in my boards I scored 94.4% .
The point that I am trying to make is that , you have ample time.Stick to standard cbse books , those are more than enough. Start preparing now, you never know , you can be All India Topper. Cheers