That's what you'd think. However, even if they told it to the authorities immediately after, they'd still be guilty for murder in the sense of 'They murdered him for no apparent reason'. In many mangas I've read, the guy who's killed or does something wrong gets off free, especially with attempted rape and molestation cases in manga (refer to 2nd chapter). So even if they told it to the authorities, they most likely would not believe Azusa if they told them he attempted rape. Why? Because in manga, authorities don't believe such young kids are capable of that in most stories. I believe that's their main reason for lying in my opinion. Either way they'll get charged, unfairly or not.
you must be naive if you think police would be understanding at all in this situation
this isn't right, at a time like this, you should always tell the truth to the police b/c you can't hide the truth forever. instead you will suffer even more when you lie like in the manga.