It will have been two years in this December since I started the lesson.
"今月１２月で、２年たったことになります。"
俗に言う未来完了を使っています。
主語は時間等の主語、itを置いています。
in this December 今月１２月で、
since I started the lesson, since I entered the class レッスン、クラスを始めてから
It will have been two years in this December since I started the lesson.
I have been studying the piano for two years as of this month.
This month marks two years of studying the piano.
ーI have been studying the piano for two years as of this month.
「今月でピアノを習い始めて２年になります」
I have been studying XX for two years as of this month. を使って表現できます。
ーThis month marks two years of studying the piano.
「今月でピアノを習い始めて２年になります」
This month marks two years of studying XX. のようにも表現できます。
ご参考まで！
日本 
カナダ 
