Do you reckon she did this alone for her mum to find and call the papers about, or that it is more likely that it is something that she and a mate had a giggle about it... I remember being 12 and a friend coming round at Christmas while we wrote up the most crazy End of Year lists, including such nonsense as which of my sister's My Little Ponies was the coolest (!!!) and grading our teachers, writing crazy wish lists and gossipy bits about others at school ("And the Flower of Hope Award goes to Mary Lacey for Wanting What She Cannot Get - Good Hair"
).
13 year olds are, well, pretty immature and often quite mean and silly too. I reckon she's done it in a free period at school or round at a mates having a laugh about it, her mother found it, they had a chat about it and a little lightbulb went on over Mum's head to say "ah, this is the sort of stuff they LOVE in the Daily Mail, maybe can make a few quid out of it". Photographer gets sent round to take pics and urges girl to scowl when really she's just chuffed she's going to be in the paper....
Most plausible scenario to me.