It all started last year, just before school broke up for the summer holidays. I had this really great mate, Jenny, but one day she started giving me dirty looks and ignoring me. Eventually she told me why. Apparently, I had been sending her lots of very nasty emails and she'd had enough: she was considering reporting me to the Head of Year for cyber bullying.

I was terrified. I had NEVER sent her any nasty emails. I rarely email (and when I do, they're always nice), but here was Jenny saying that I had sent her vicious emails. It didn't make sense.

She printed off a copy of some and when I read them I was horrified. My name and email address were above these sickening, really awful emails. How could this happen? I KNEW I hadn't sent any mean emails, but then... what were these? I couldn't explain it.

Bullied because of something I didn't do

Jenny really believed the emails were from me. She started being mean to me and stopped all my friends from talking to me. They really bullied me and made school life so tough.

Jenny even told her mum, who had a real go at my mum. I told my mum the complete truth: I hadn't sent any of these awful emails. Mum believed me and stuck up for me.

That's when Jenny went to the Head of Year.

The truth comes out...

I was so scared waiting to be called in to see the Head. I knew I could be suspended or, worse still, expelled for something like this because I had no proof it wasn't me.

Then I suddenly remembered: I had written my email address and password in the back of my school planner a few months after I had set up my account, so that I could access my emails during free time at school.

I had accidentally left my planner in the ICT room but when I went back to collect it, it was gone. I didn't think anything of it at the time, I just thought it was annoying. Now I realised that someone must have found my planner with my email info inside and was accessing my account! I was distraught - they were purposefully trying to destroy my school life.

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In the clear

I explained all of this to the Head of Year, who told me this had happened before in school and that she believed she knew who the culprit was! I couldn't believe it. Thank goodness I had remembered about losing my planner.

The Head interrogated her suspect and eventually got her to admit to using my account to send horrible emails to my friends. It was a relief to have my name cleared.

But the biggest shock was yet to come...

Shock revelation

This girl? The one who'd been sending these emails? The one who turned my life upside down? Turns out it was my BEST FRIEND, Amy. It was like something from a movie. But not a very nice one... I couldn't believe it.

We've sorted it all out now and although I'm friends with everyone again, including Jenny, I'll never be able to forget what my so-called best friend did to me.

Be careful with your personal data; NEVER take it to a public place and use it there. It's such a stupid thing to do. I did exactly this and look what happened. In some ways, I guess I was lucky... that information could have been used for a lot worse.

I've learnt a valuable lesson, but unfortunately I learnt it the hard way. I hope you don't have to.

Slink's advice on keeping your identity safe:

Never leave any personal info (such as your email address, home address, phone number etc) lying about or displayed on your social networking site where anyone could see it.

Check out Sorted for loads of practical advice about keeping your info safe and what to do if you do end up in a situation similar to Cassie's.

Also, why not have a little look at Slink's guide to staying safe online?