I think a lot of us get where you are coming from, but they do have a lot to take onboard the first couple of months in Year 7. They are trying to be independent as the school encourages, but at the same time have a lot of things to think about, being in a different environment with different expectations, having to make new friendships and not knowing what other girls expect, ie can I be myself or have I got to look more grown up, having to work out how to get to every lesson in a large school, having to take onboard extra homework and perhaps putting extra work in as they don't know what level needs to be worked at.
She will learn from the detention and probably never forget homework again. My daughter had one detention in the first term and I certainly wasn't pleased (she drew on someone's collar and I later found out is was because she wanted the girl to leave her alone and she was too scared to tell a tutor). It was delayed a month due to sickness and then Xmas she spent all that time worrying about it and kept waking in the middle of the night - as soon as she done it, she went back to sleeping throughout the night.
Perhaps, you could check her bag after she's gone to bed. If you think something is missing, just gently ask her if she's remembered the brilliant piece of homework she's done which is due to be handed in, or mention you've notice her workbook on the table (or wherever) and does she need it. You might get a negative answer, but she does still need your support.
The positive thing in this, is that she wanted and still felt able to phone you and tell you what had happened.