[{_id:"1",school:"jhc",academic_year:"2020"},{_id:"2",school:"jhc",academic_year:"2021"},{_id:"3",school:"jhc",academic_year:"2021"},{_id:"4",school:"khc",academic_year:"2020"},{_id:"5",school:"vghs",academic_year:"2020"}]

db.collection.aggregate([{"$group":{"_id":{sc:"$school",ay:"$academic_year"},total:{$sum:1}}},{$group:{_id:"$_id.sc",total_students:{$push:{_id:"$_id.ay",total_students:"$total"}}}}])