Is it legal for school to terminate my services if i am pregnant?

Govt. Schools and Government Institutions shall be governed by CCS (Leave) Rules 1972 and the provisions of Maternity Benefits Act, 1961 shall be applicable to them. 

The Hon'ble Supreme Court in Municipal Corporation of Delhi v. Female Workers (Muster Roll) and Another, had held that the provisions of the Maternity Benefit Act, 1961 are applicable to the women engaged on casual/contractual basis or muster roll basis and not only those on regular employment.

Hence reading the provisions of the Act with the Rules, you are entitled to maternity benefits such as leaves. 

Therefore, when it comes to contract renewal as well, they are under an obligation to consider your renewal after your pregnancy and they cannot  refuse your contract renewal on the terms of pregnancy. You can approach the Central Administrative Tribunal or the Delhi School Tribunal for relief in case the School refuses your contract renewal.

Also, as you have mentioned that they have not given any reasons for your non-renewal of the Contract, this can be challenged as arbitrary and unreasonable.

The Supreme Court of India has categorically held that it will interfere in cases of contractual employment as well if the decision of the Management is arbitrary. Therefore, even though your term is over, you can challenge the decision of the management of non-renewal of the contract and they will be under an obligation to show valid reasons as to why your renewal of the contract was not done.

The fact that you have a clean record shall work in your favour.