Excellent - if you want her to cut the crap and change her behaviour, then you need to get off your booty and start sorting that out.
Follow up with both her head of year and 12yos head of year. Today. (Follow up that message by calling the school back and making sure the sec knows it is imperative you speak with both heads of year today about a safeguarding issue at home). You should also consider calling the 6yos HT.
Call gp and make appointment for you to discuss your own my and CAMHS referral for dd - urgent appointment. Tell receptionist you have been having fleeting suicidal thoughts.
Those two will do for now. If you don't want to call SS yourself, then that's fine - having those discussions with school and gp should ensure that SS are brought on board for support in any case.
I do get that sometimes it is appealing to check out and just let it happen around you - whatever will be, will be, and all that. But you have now been checked out for three months, so it's time to get back in the game and back in control. With the catalyst being that your 12yo was assaulted today. You actually have no option but to act now.
If, as you say, this is a case of a dd that is capable of behaving, but for whatever reason chooses not to, the appearance of a mother who is suddenly back in charge, and a support group of responsible adults who are all working towards the same goal, may lead to the required change in her behaviour as suddenly the whole show will be blown wide open and she will no longer be the goody two shoes A student. It may be something of a relief to her. You can then all work towards appropriate resolution.
Or, you can do nothing and huff and puff, come up with a myriad excuses, change nothing, and continue to have one child assault others and safeguard no one.
Parenting is something of a choice. You have chosen to check out for a few months out of sheer exhaustion, and that's fine. But ultimately, you have now got to get your head back in the game and check right back in.
Or, give up totally and hand them all over to SS. But that requires action too.
But literally doing nothing is not an option. It is an active choice what to do next, but just sitting back and watching while your children assault each other is not a choice you can make.