D., first, I want to say you are doing the right thing to halt all TV and computer time and to say no to the party; just be sure that your withdrawal of all TV and computer is total and lasts for a very long time, because this is a very serious offense; a week or two of doing without "screen time" is not enough here. I also might ground her entirely for a long while -- no play dates, no friends over, no phone calls to friends, etc. She needs to understand that what she did is very serious. It's about lying and lack of respect, but does she also understand that it's about her own safety? During that before-school band time, where was she playing? Who was she with? She could have wandered off or anything could have happened -- and you would have thought she was safe at band and the band director would have thought she was safe at home, having quit band. This needs to be brought home to her so she understands.
On the bigger issue: You also are right to be as concerned as you are. Her behavior is more like that of a middle schooler or high schooler than an elementary level child. Ask yourself: How did she know she could deceive you? Why did she believe she would get away with it? And how did you finally find out the truth about band -- because she finally confessed out of some sense of guilt, or because she confessed only due to fearing being caught, or because you found out another way, such as the band teacher contacting you? If SHE told you, that is at least better than hearing it another way and having to go to her and say, "I know you've been doing this...." when she was planning to just continue her deception. I would let her know that this one big deception, and the little deceptions of hiding homework, now have you questioning everything she says and does; you cannot trust her right now, about anything, so she will have to stick close to you and account for everything -- and she has brought that on herself. Tell her you want her to earn back your trust because you DO want to trust her, but she has broken trust and must earn it with truthfulness.
D., the band issue combined with the lying about homework indicates she does not worry or care about displeasing you or letting you down. So building some respect for you is vital. So is building respect for her school commitments.
It is great that you are already working with the teacher! That needs to increase. I would immediately ask for a meeting between you, the teacher and the school counselor about this; give them every detail about the band issue; and say that you need some outside help to nip this in the bud now, because if she is doing this at nine, she will do much more and much worse ditching as she gets older. Ask the counselor and teacher for specific ideas on what to do (discipline) and what to say (ways to teach her to respect you and to respect school) and also ask what they think may be the root of this.
For instance, she may be a child who operates (many do) on the principle of "I want what I want, and I want it now," which you need to fix or she will not only lie to you, she will drive off friends eventually and alienate teachers to the point they won't want to keep helping her. She may need more rules and chores (with rewards if she does well!); she may have friends who are encouraging this behavior (so she must get some new ones); she may be afraid of failure at school or in band, so she says "I don't want to get up early" when the truth is she is just scared to death of band because she thinks she's bad at it. That's a self-esteem issue that would need work. There are a LOT of possible motivations to explore here. And a lot of new rules to set up, and new systems for rewards and discipline that you must use, consistently, every single time.
As a single parent you don't have another adult around to help enforce things, so you will have to do it solo and that will be tough. I would ask the counselor about how to get some professional counseling for you both as a family and also some parenting counseling for yourself, to help you map out the behaviors you expect and the rewards and consequences of her behaviors, and to help you be consistent. It is tough to find time for meetings and parenting classes but if you do it now you will probably have fewer problems later -- this really is your wake-up call to do something immediately. Please update us here!
