How common is sexual harassment in secondary schools?

Personal experience from 25 years ago, it can be a huge issue. And not just harassment, assault too.
Hopefully it's better now but I have boys so don't hear the realities from a female perspective

Oh, and I went to an all girl's school too, and remember welcoming any kind of male attention outside of school, not all of it positive.

In my workplace (a university, and before that an IT consultancy) we get mandatory equality and diversity training which makes it very clear what (sexual, racial, etc) harassment is and how we should call it out if we witness it or experience it. My 2 boys have both had similar training at their mixed secondary.school. I don't know if all schools do it yet, but I'm pretty sure that's where things are moving to with the new compulsory SRE curriculum. See info here: assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/781150/Draft_guidance_Relationships_Education__Relationships_and_Sex_Education__RSE__and_Health_Education2.pdf

@Stokey I think you are raising some good points there.
I have always been wondering whether girls schools give a false sense of security and strength which will only lead to shock and disappointment when the girls get through the mixed world.
And equally thinking that if the boys are in school with the girls they are more likely to know how to behave, and a fair amount happens in a controlled environment-as opposed to parties etc
Everyone learns by being together in a supervised environment, they are committed too through studies, with rules, codes etc

Sexual harassment exists everywhere in life. Reality is you can't protect your girls from it (I have two) but you have to educate and empower them to know what's right and wrong and to speak up when they are faced with it and call it out.

We need real leadership in all schools to call out sexual deviant behaviour and we need male teachers and voices to stand up here.

@Justajot I also have two DDs in primary. I was having a conversation about whether co-ed or single sex schools would be worse for sexual harassment. Obviously no boys in single sex but might that mean girls from a single sex school are more likely to be targeted outside school? Whereas in a co-ed school, teachers would be more aware and clamp down. Also would a boy be less likely to harass someone that he has to have classes with the next day? Really not sure.
I went to a girls school too many years ago (& had my fair share of harassment at parties etc).

I’m an assistant head at a large mixed comp in outer London. In terms of sexual harassment for us as a pastoral issue, it happens but it’s less common than other types of bullying behaviours. The last couple of major incidents we had both related to distribution of intimate pictures.

There has been recent changes to the Keeping Children Safe in Education to highlight an increase in “upskirting” but it’s not something I’ve seen personally.

Not common at all.
You might not get a balanced view asking on an Internet forum though - people are more likely to reply if they have had a bad experience or if they have a particular agenda. You would need to research some statistics yourself (or do a random sample ).