When I was 15, a boy at school slapped my bum so hard that I had a hand-shaped bruise there for an entire week.
I don't remember much from that day but I remember the impact of the slap hurt. A lot.
I also know that there were a lot of boys hanging around when it happened and that they all laughed. I might have even laughed too. But inside I was dying of embarrassment.
When I read this morning that one in five British women experienced sexual harassment at school, I immediately couldn't believe the number was that high.
The report on sexual harassment in schools found that of 2,000 British women, 22% experienced some kind of harassment at school including: sexual touching, groping, flashing, sexual assault or rape.
It was reading that statistic which triggered the memories of my bruised bum cheek - and not being able to sit down properly for days - to come flooding back to me.
I felt compelled to write about my own experiences because for so long I hadn't really considered that episode at secondary school as "sexual harassment". For years I'd thought of it as one guy's (really bizarre and somewhat painful) way of flirting.
And I'm not the only one who has brushed off sexual harassment during their school days.
When I mentioned I was going to blog about the issue, one of my colleagues immediately mentioned the time that someone lifted her skirt and pinched her bum at school. Another colleague had a guy grab her hand and place it on his crotch during a lesson. At my school, boys would tip water over girls so that their shirts went see-through in summer.
None of us reported it, but all of these are examples of harassment.
It's disgraceful behaviour and something that, in an adult environment, would never in a million years be tolerated.
But for some reason at secondary school it's rife. It happens, probably on a daily basis, and I can bet that there are a lot of girls out there who don't realise that what they are being subjected to is sexual harassment.
Heck some of the perpetrators probably don't even realise what they're doing is so inappropriate.
In my case, I was a late developer and nobody had ever really paid me any attention until I turned 15. Then, all of a sudden, boys developed an interest - almost overnight. And that's when the inappropriate behaviour started.
Let's face it, nobody really teaches you about dealing with this kind of thing. You're taught maths, English, music, art, geography and lord knows what else. But it's a complete minefield when it comes to dealing with the opposite sex.
And while my bruise healed, for other girls the mark(s) from sexual harassment can last a lot longer.
As Tanya Barron from Plan UK, who issued the figures on girls and harassment at school, puts it: "Unwanted sexual contact can affect a young girl's self-esteem and educational achievements."
It doesn't take an expert to know that what is going on in schools is so very wrong. In my opinion, as someone who has been there, done that and got the unsightly bruise, I believe that young boys and girls need to be told exactly what counts as sexual harassment.
Boys should be taught about how to respect and treat women. And I know most young men already do, but it's clearly not enough. If more boys respected their female peers, one fifth of young women wouldn't be experiencing sexual harassment at school.
Teenage girls need to be know a) what is the best way to deal with such an issue and b) who is best to report this kind of problem to. They also need a safe and non-judgmental place to go and talk about harassment.
Meanwhile boys who witness sexual harassment should be encouraged to speak out and stand up for their female counterparts.
PSHE education, which is not part of the statutory national curriculum, should be made compulsory covering issues such as consent, sexual health and dealing with harassment.
The statistics speak for themselves, it needs to be on the curriculum.
If all of this were to be explained to teens properly, and the seriousness of the issue was to be hammered home, it might just save another girl from a bruised bottom. Or worse.