＞We, DMM On Line English Conversation School, had not given you customers a lesson until 2015 during the Christmas season and the period before and after New Year's Day, but we have heard a lot of valuable and instructive advice from customers.

On Lineって何よ？
had not given you customers a lesson これも何？目的語が３つもあるの？って思う。
the period before and after New Year's Day, この回りくどい表現は何？しかもthe periodは副詞？名詞違うの？
have heard a lot of valuable and instructive advice 　heardはおかしいでしょ。a lot of もカジュアル過ぎないか？

ってか、なんか英語が変。
この英語で「素晴らしい」なんて言われてたら、いわゆるゴマメ扱いされてるはず。