db={"school":[{classes:[{name:"A",students:[{name:"John Doe",age:15,note:9},{name:"Jane Doe",age:17,note:8},{name:"James Doe",age:15,note:7}]}]}]}

db.school.aggregate([{"$match":{"classes.name":"A"}},{"$unwind":"$classes"},{"$match":{"classes.name":"A"}},{"$unwind":"$classes.students"},{"$group":{"_id":"$classes.students.age","class":{"$first":"$classes.name"},"sumNotes":{"$sum":"$classes.students.note"}}},{"$project":{"_id":0,"class":1,"age":"$_id","sumNotes":1}}])