I was in year 10, so around 15 years old. A girl in our friendship group claimed to be pregnant, and devastated. Cue lots of rallying around her, trying to figure out how to tell her parents, keeping it very, super secret so nobody found out. Tears and hugs, all the rest.
...Only she'd confided to me shortly before this that she'd never kissed someone, let alone slept with anyone, and didn't have a boyfriend... so I smelled a big smelly rat. I tried to point out that it all seemed like fucking bullshit but nobody listened to me.
To her credit she was incredibly convincing. She'd come to school for a few weeks without make up (this was in the era of Dream Matte Mousse two shades darker than your skin, and the over application of such, so she stuck out), run off to the loo to 'be sick' and generally act exhausted and depressed with her situation. All very believable if you're fifteen.
This goes on, and on and on and she still has everyone 100% convinced a month or so later when she's supposed to be 12 weeks gone.
It was finally revealed as a pile of shit when she brought in a scan photo, from the scan she supposedly managed to obtain without telling her parents
that she had printed from the internet with someone else's fucking name on 
After that she lied about being a bikini model for a kit car company, having cancer and being addicted to heroin as well as loads of other ridiculous shit.
She's done pretty well for herself and is married with kids. I see her in town sometimes and wonder if she's still a liar liar pantsonfire.