In a similar situation, I did this:
Sit DC down at a time when you both have a good hour to spare. Make it as pleasant as possible. Cosy seats, blankets, a mug of tea etc. Tell her you read a message that she probably wishes you hadn't but you are very glad you did. Show loads of sympathy.
Tell her you are really proud of her for being so analytical about her problems, and that being able to recognise them is half the battle. I am so impressed by her. That is very emotionally intelligent. I hope she knows that. Then tell her that you and teachers and counsellors are adults who can truly help but it's maybe not a good idea to put pressure on a friend as they are emotionally not mature enough to offer support and may have secret issues of their own.
Ask her some gentle questions and listen loads. Give lots of encouragement about how insightful she is and encourage her to get the whole lot off her chest however trivial some of it seems. It's likely she'll get upset, so have tissues and a blanket to hand, to comfort her. And hugs, obviously.
Help her work out a plan of action. That could include an ADHD assessment. You could also discuss how sugar highs and lows can get people to behave in a hyper manner and suggest she cuts right down, replacing sugary snacks with plain savoury ones. Find out which teachers she likes most and contact them first, as well as her form tutor, Head of Year, head of Studies and Counselling team.
Remind her that it's very unlikely teachers dislike her but they probably struggle with that behaviour as it makes it harder for them to do their job.
DS2 has ADD (not ADHD), and there are loads and loads of simple tips that help at that age. She could get permission to have her phone on her and take a photo of work on the board. She can ask for all homework to be emailed to her by the teacher so it's in writing somewhere she can't lose it. At DS's school the counselling team were so brilliant, but I get the impression we were very lucky as other friends have had to fight for the right support.
She may not see it yet but having that sort of insight into her behaviour, and having such a clear, strong will to improve it, and to have the vision for how she'd like to behave... if that were my daughter I'd be bursting with pride at how self- aware and emotionally mature she is inside, even if the outside hasn't caught up!
She's only in Yr 7. She'll get this sorted, definitely. [flowers to her]