The Largest school in terms of pupils is
the City Montessori School in Lucknow, India, which had a record enrolment of 39,437 children on 9 August 2010 for the 2010-11 academic year. The school admits boys and girls between ages two and five, who can then continue their education to degree level. In 2002, it won the UNESCO Prize for peace way since Jagdish Gandhi and his wife Bharti first opened it in 1959 with a loan of just 300 rupees ($63, £22). Then it had a grand total of five pupils!
(The above is quoted from the Guinness Book of World Records 2013 edition. The current student strength is over 47,000 pupils)
The 2002 UNESCO Prize for Peace Education has been awarded to the City Montessori School, Lucknow (India) in recognition of its efforts to promote the universal values of education for peace and tolerance at a time when these values are increasingly being challenged. (see link)
The largest school by pupils is the City Montessori School, Lucknow, India. (see link)