public static async Task mainAsync()
{
var db = new MongoClient().GetDatabase("school");
var collection = db.GetCollection<BsonDocument>("students");
var list = await collection.Find(new BsonDocument()).ToListAsync();
var newList = new List<BsonDocument>();
list.ForEach(data => newList.Add(dropScore(data)));
await db.DropCollectionAsync("students");
await db.CreateCollectionAsync("students");
await collection.InsertManyAsync(newList);
}