My DD is Y7 and it has been a really difficult transition. Frankly the first half term was bloody awful.
They have so much to remember - homework every night, when to hand it in, what classes they have the following day. DD was in a state of high anxiety about punishments and detentions, losing/forgetting her log book/forgetting to get it signed/PE/food tech ingredients/ it goes on and on. She's also just started her periods and has to remember pads etc. too. It's getting a bit better now, but like you, OP, we almost missed the Christmas concert but we were lucky there were some tickets returned.
The only way I've found is to get a good routine going, and be (gently) on her case all the time about it. DD comes home, has a drink and a snack, does her homework. Then the logbook gets checked for the following day and her bag has to be packed the night before.
This has to happen before tea, TV, ipod, chatting with friends or anything else. We have a special crate where everything school-related is kept, so anything I find lying around the house gets put in there.
She hates me poking around in her bag but I do still have a good sort through now and then (when she isn't around) otherwise it gets heavier and heavier! I did this just this morning while she was upstairs getting dressed.
Gradually we're getting there. Yes they are old enough to "take responsibility" but some are better than others at different things at different times. At that age sometimes they're 11 going on 25, other times they are 11 going on 6. Like everything else, organisation is something you have to teach them - and keep teaching them - until it becomes a habit.