A complicate mood

In my opinion, we should help our friends from being trouble. Because, they always got your back and help you as possible as they can. However, this time is different, I didn't help my friend and even get him into a trouble. Everything was started from last week, one of my good friend drawn some ugly things on another student's stuff. I saw it, but I didn't tell him to stop. Today, teacher asked me, did I draw those stuffs or not. Of course, I would say no because I didn't do anything.  Afterwards, the case was getting worse. The victim's mother was calling school and complaining what have we done to her son's stuff. When I noticed the situation was wrong, I  told teacher exactly who did it. I felt so guilty and depress. Even though I should tell the true, I didn't protect or avoid my friend get into trouble at first time. Now, I hope this case won't affect the relationship between him and me, and we can still be good friends as well as past time.