When did this happen? You say they were going to contact you this week as though it happened the week before. Then you’ve waited until after 5 pm on a Friday to start seeking external opinions. Did you not call today at 2 to find out why they hadn’t been in touch?
I think you’ve been a bit too trusting and passive. Understandable if the school have otherwise been good and said all the right things. But schools are busy and their focus and priority is almost never on the individual actions of children unless those actions are directed at the school in some way or there’s external pressure. You need to be the squeaky wheel.
Over the weekend, report it to the police. Ask them to investigate as the school have failed to and you do not feel they are able to take it seriously enough since it is a serious offence: Offences against the person act 1861, Section 24 - administering a noxious substance with intent to injure, harm or annoy another person. It has a maximum sentence of 10 years (though this would be at the lower end of the scale and not likely to get any sort of custodial sentence). Because they planned it and acted together, after a campaign against your DD (the bullying), it is more serious than if it were one girl just acting on a whim.
Email the school to let them know you’ve done this as you no longer have faith that they have are able to respond with the seriousness that this assault requires. Ask them to cooperate with the police and tell them you would like a meeting with the [senior leadership team members in charge of discipline and safeguarding lead - look these up on the school website] or the head teacher to discuss how they will keep your DD safe from further assaults and any retaliation for the rest of the school year.
Look up their bullying policy and have it with you to hold them to account, but do not let it over ride the police investigation. Actions should happen in parallel, especially in regards to them protecting your DD.
On Monday, separate to this communication, put in an official complaint about how the school have handled it so far (not about the assault, just about the lack of communication) as they did not get back to you when they said they would.
Make sure there is a significant paper trail that you can go back to if they try to brush things under the carpet.