[{"_id":"class_a","students":[{"_id":"1a","name":"John","grades":[]},{"_id":"1b","name":"Katie","grades":[]},{"_id":"1c","name":"Andy","grades":[]},]}]

db.collection.update({"_id":"class_a"},{$push:{"students.$[students].grades":"A+"}},{"arrayFilters":[{"students._id":{"$in":["1a","1b"]}}]})