According to Official Guinness Records,
The largest environmental sustainability lesson consists of 2,289 participants, achieved by Larsen & Toubro, Water and Effluent Treatment IC (India) at Chennai, India, on 16 March 2018.The lesson was organised for college students and focused on saving and recycling water.
For a complete list of 2018 records, please visit 2018 Guinness Records in India.