Sir all Educational Institution are not covered under the Mega Exemption Notification list i.e. Notification No. 12/2017 - CT (RATE ), dated 28.06.2017.
So if your Aggregate Turnover exceeds Rs. 20 lakhs, then you have to apply for GST Registration. Aggregate Turnover includes EXEMPT SUPPLY and exempt supply as defined under Section 2(47) of the CGST Act 2017 includes NIL Rated, Zero Rated.
( Turnover means Taxable+ Exempted + Nil . right?? )
Yes... You are right.
But, If Your income is as per the above formation/details then GST registration is required/must if total turn over exceeding Rs 20 lakhs (Turnover means = Taxable+ Exempted + Nil ).