I think you have to talk to the older boys/DD. This isn't funny.
I would make the point that showing pornographic material to under 16s is a criminal offence. Them doing it is no different from some perve doing it. If they are planning on giving copies to their brother, who else is passing this porn around at school, are other underage boys seeing it? It could be seen as grooming. It is not appropriate behaviour. Some of those older boys could end up with a criminal record and on the sex offenders register. That might make them all take it a bit more seriously and not see it as a big joke.
I might have a word with the school and let them know there's a porn collection being passed around the boys.
I'd also want to talk to them all about porn in general. I was never particularly anti-porn in the past, but current porn is truly abusive and awful. Without going into how, I saw a lot of porn mags when I was a kid. Didn't do me any harm, didn't effect my sex life, because what was depicted was pretty normal sex (with any kinky stuff appearing to be consensual and not abusive). I've also seen porn movies from 80s and 90s. I wouldn't want a kid to see those (or mags of course) but again, nothing I would say would be particularly traumatic. I've also seen some of the free internet porn available now. It is shocking. Very violent and abusive as a matter of course and no attempt to show anything like normal sex. Women faking enjoying being gagged during oral sex until they are vomiting, and then made to drink the vomit, and forcing a horrible smile throughout it all. Women being hurt. Anal sex as a standard and they are lucky if it's only one penis. Anal 'stuff' that deliberately creates a prolapsed rectum as 'sexy'. Jeeez. I'm never looking at porn again.
Studies are showing that teenage boys and girls are developing very fucked up ideas about sex. That they believe these things are normal. That girls who don't enjoy this type of sex have something wrong with them. Or that of course girls don't enjoy it, but that doesn't matter, it's for the man to enjoy and the girl to service him. I'd be desperately worried about even your older children.