The largest school in terms of pupils is the City Montessori School in Lucknow, India, which had a record enrolment of 39,437 pupils on 9 August 2010 for the 2010-2011 academic year. The school was founded by Mr. Jagdish Gandhi and his wife Mrs Bharti Gandhi. The pair launched their first campus on a borrowed capital of 300 rupees, housed it in rented premises and began with only five students. The school admits boys and girls between ages 2 and 5, who can then follow through their education to Degree Level. The school was the recipient of the 2002 UNESCO Prize for Peace Education.
(The above is quoted from the Guinness Book of World Records 2013 edition. The current student strength is over 50,000 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)