How much does it cost to build a school in India?

It completely depends, where you want to build, in Major cities like Delhi, currently Gurgaon, you will not get a land less than 50 Million. Building would cost same for good high school, another 2-3 million will go in affiliation, permission (off record)

so approx 100 Million.

One of international chain is planing to open the same with investment of INR 1000 million starting with 4 metros only.