Question:

I fell down the stairs this morning, pulled a muscle in my leg and I can't come to the lesson today. :)

Answer:

今朝階段から落ちて肉離れを起こしましたので、今日の授業には行けません。
kesa kaidan kara ochite nikubanare wo okoshimashita node, kyono zyugyo niwa ikemasenn.