I agree with the comments about there being pressure on kids to "get it right" when it comes to testing. But I also think they may want to say blood glucose levels are better than they really are just to avoid the perceived "fuss" a high/low BG can sometimes cause . Given the choice between saying BG is 12 and having a "why do you think that is?" or "shouldn't you take a correction dose?" conversation or saying BG is 6 and being able to move on with whatever they're doing, I could understand a teenager opting for the second choice. Even more so if teenage hormones mean BGs are all over the place, so the teenager has no idea why levels are out of whack. Also, you sound like a caring parent, so perhaps your daughter knows how much you want her BG levels to be controlled and is either altering the readings/using control solution to give lower readings, or deliberately testing only at times she knows she has "good" BGs, because she doesn't want you to worry.
Regardless of what the meter is saying and whether it is accurate/altered, there is no question that your daughter's HBA1C is relatively high. I would expect the HBA1C to be her DSN's concern, rather than whether or not your daughter is changing her readings. I think the suggestion of getting your daughter to read some of the forums for kids with diabetes is a good idea, as it may help to give her ideas on helping get better control. The other thing you might want to try is actually stepping back from her diabetes management a bit, and perhaps letting her have 1:1 time with her DSN without you there to discuss her relatively high HBA1C and some ways to manage it? You said that you upload the readings and had the conversation with the DSN, but perhaps it would be better for your daughter to manage the upload and speak to her DSN directly? Most teenagers are fairly independent, and ultimately it is what your daughter does that will most influence her BG levels.