According to Official Guinness Records,
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.
For a complete list of 2010 records, please visit 2010 Guinness Records in India.