[{"student_id":5,"class_id":18,"scores":[{"type":"exam","score":73.04238861317688}]},{"student_id":5,"class_id":8,"scores":[{"type":"exam","score":22.38732080941065}]},{"student_id":5,"class_id":0,"scores":[{"type":"exam","score":43.64758440439862}]},{"student_id":5,"class_id":16,"scores":[{"type":"exam","score":33.39752665396672}]},{"student_id":5,"class_id":30,"scores":[{"type":"exam","score":73.48459944869943}]},{"student_id":5,"class_id":19,"scores":[{"type":"exam","score":15.36563152024366}]},{"student_id":5,"class_id":23,"scores":[{"type":"exam","score":21.58296008740177}]}]

db.collection.aggregate([{$match:{student_id:5}},{$unwind:"$scores"},{$match:{"scores.type":"exam"}},{$sort:{"scores.score":-1}},{$group:{_id:"$student_id",doc:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:"$doc"}}])