Hey shiv

First of all, all the best for your future ahead. Don't take too much tension or pressure on yourself just do great amount of hardwork in present so that you can enjoy sweet fruit of your hardwork in future. 

To get admission in IIT and NIT you just have to score good rank in JEE(MAINS) and JEE(Advanced). You can easily get into NIT if you scored well in JEE(MAINS) and scored atleast 75% in class 12th boards. It has nothing to do with from which school you passed you class 12th. 

for getting admission in IIT you have to score good rank in JEE (advanced). Last year, in 2018, the good score for JEE Advanced was 243 out of 360. you have to qualify rank list of JEE advanced 2019.

If you are not clear yet you are free to ask anything.