[{"name":"School 1","classes":[{"name":"Class 1","students":[{"name":"Student 1"},{"name":"Student 2"}]},{"name":"Class 2","students":[{"name":"Student 3"},{"name":"Student 4"},{"name":"Student 5"}]},]},{"name":"School 2","classes":[{"name":"Class 3","students":[{"name":"Student 6"}]},{"name":"Class 4","students":[{"name":"Student 7"},{"name":"Student 8"}]},{"name":"Class 5","students":[{"name":"Student 9"}]}]}]

db.collection.aggregate([{$project:{classes:{$map:{input:"$classes",as:"classes",in:{name:"$$classes.name",studentsCount:{$size:"$$classes.students"}}}}}}])