Normally that leads to a police hunt, appeals for information, and hopefully finding her safe and well.
A child goes missing with an adult, and alarm bells normally ring. If the adult is a teacher, the worst is soon presumed.
But a fifteen-year-old goes missing with her teacher and people queue up to say this is not a problem. That there is little difference between fifteen and sixteen, that these things happen, who hasn't dreamed of either a school girl or a teacher, and idiots say "so what".
Physically an adult but emotionally still a girl, she is regarded as a sexual being with the requisite experience and judgement to choose her lovers wisely. If any blame is apportioned, it is hers.
Then we have an official report about something that happened to another fifteen-year-old. She went missing, too.
She went missing quite often, and eventually told the adults who were responsible for her she had been abused.
She said she had been sexually assaulted by more than one man, and asked for help.
The social workers to whom she complained did not tell the police, and did not take her seriously. Instead they told her parents she need to be given "boundaries", because if she was having sex it must have been her choice.
No-one presumed the worst. Yet the worst was what was happening.
The abuse - carried out with threats of violence, psychological pressure, drink, drugs and bribes - continued and the girl became pregnant.
No alarm bells rang.
A year after her first complaint she went back to the authorities and asked them to help again. The police gathered enough evidence to charge several men, and found other girls had been victims too.
A month later our girl withdrew her statement, on the grounds she had been threatened by her abusers and she did not believe social services would protect her if she went ahead.
Normally if a child tells the police they've been threatened they take notice. Usually when a girl says she's too frightened of an attacker to give evidence the Crown Prosecution Service will proceed against them anyway, to get them off the street.
Not in this case. The CPS dropped the charges despite having three other victims who could testify, and social services ended their involvement with the girl. She was no longer, in their minds, "a child in need".
She was described as an "unreliable witness", which is apparently her fault and nothing to do with the fact someone who's been systematically abused for years, plied with drink and drugs, bullied, terrified and raped often winds up a bit screwy.
She had the body of an adult so it mattered not in the least whether she was able to make choices in the same way as a 30-year-old. It was her fault, she was old enough to know better, and they thought 'so what'.
The girl self-harmed, she used drink and drugs, she ran away. No-one gave a toss.
A year later another man was arrested because of the girl's evidence and he said enough to the right people for local police to launch a full-scale probe in to the organised sexual exploitation of children in Rochdale.
In May - four years after that fifteen-year-old first said she'd been abused - the gang was finally jailed.
There is of course a world of difference between the case of Megan Stammers, a schoolgirl who's run off with her 30-year-old teacher, and the girl known as Suzie by the Rochdale Safeguarding Children Board whose report into exploitation has today revealed a shocking series of blunders which allowed rapists to attack a string of vulnerable girls for years.
There are different causes, villains, methods, and official mistakes.
But at a fundamental level these two fifteen-year-old girls have been treated by the world at large as being much the same - as a kind of prostitute, someone who knows all about sex, makes a choice and must accept the consequences.
Because they've entered puberty, they can be trusted to go shopping on their own or cook a meal, then whatever happens to them is, somehow, seen as their entirely-independent decision.
Have you forgotten what it was like to be fifteen? The feeling that no-one understands you, the self-loathing, the wish to fit in, to escape, to grow up? The fact that you can get away with buying booze but you don't really like it and drink it just to show your mates you can? The way people talk about sex and you laughed and joined in because you didn't want them to realise you had no idea what they were on about?
Yes, your body's doing all kinds of interesting stuff. It sprouts hair and wobbly things which you half want to hide away and half want to boast about. It doesn't mean you have much of a clue what to do with it, or more importantly what you ought to do with it.
Peer pressure has you by the throat, hormones twist up your insides, and as much as you want to be an adult you're not quite there yet.
When I was 15 my gran died, and I held my mum when she cried and felt very old indeed. I grew up a lot in those few weeks, but it didn't make me an adult. I acted as though I were, but I still cried like a baby when I thought no-one would notice.
Perhaps part of the problem is our attitude to when childhood starts and finishes. We say you can have sex and join the army at 16, drive a lethal metal box on wheels at 17, but you can't make your own mind up about things or vote until you're 18.
Yet the age of criminal responsibility was as low as ten for the two boys who abducted and killed Jamie Bulger, and they were tried as adults despite the fact they probably needed as much if not more care than Jamie's grieving parents.
Pubescent teenagers were jailed for taking part in last year's riots, yet when girls of similar age are married in minority cultures it's widely regarded as wrong. Perhaps we ought to just set one age for all those things, and end the confusion.
Too many people say 'so what' and leave it there, when they ought to follow it up with 'are we doing about it?'
A fifteen-year-old can be raped. They can be abucted, they can be cajoled, coerced, persuaded just as any child - or any adult, come to that - can be. They can be very mature for their age, they can be from nice homes or bad ones, they can put their personal grief to one side to comfort their mum, but it's just a step on the road to being grown-up.
Prostitutes can be raped. People who are drunk can be raped. People who think he loves them can be raped. A girl of 15 years and 364 days is not magically better at making decisions about boys 24 hours later, or 24 years later for that matter.
Childhood ends at different points for everyone, but it's generally when you realise the world can be a horrible place. That moment should be put off for as long as possible, and when it happens there's ideally an adult nearby who can help you adjust to the idea, not exploit it for their own ends.
Not just shrug, and say 'so what'.
Because what would that make you?