When school send home a list with any dates or letters with any important information on, I always take a picture of it with my phone as well as trying to remember to enter the dates on my calendar (with a column for each member of the family plus one for general stuff), in my diary and on dh's calendar. That way if you need to refer to it or look up dates you usually have your phone with you to check on the info, rather than needing to find the piece of paper which has usually disappeared regardless of how organised you try to be with them.
I always try to have everything ready for each child the night before - they are in different schools. I have a big bag for each of them (the sort of jute or recycled plastic ones that they have in supermarkets) into which goes their school book bag and anything else (sports kit that comes home, library books, lunch money, jumper, etc) that needs to go into school. Shoes are put by the bag, coat on top.
Lunch bags are made up as far as possible the night before, even bits that need to be kept cold are done and left in the fridge ready to pop into the lunch bag in the morning. As soon as they are done (usually while the dc are eating their breakfast) the lunch bags are put into the big bags in the morning.
Then when it comes to going out in the morning, I only need to pick up one bag per child and not have lots - especially at the start of term when there are lots of bags going in so it is easy for one to get missed. I use the same bag when picking the child up so all their junk stuff can be thrown into one bag and I'm not left trying to hold onto armfuls of stuff and a toddler who likes to make a dash for freedom, I've just got one big bag to hold on to.
I use good laundry markers to mark clothes - and keep several dotted around the place so it is always easy to remark the clothes if the name looks like it is fading. I have one in each car, one on the landing windowsill near the airing cupboard, one hidden at the top of each child's wardrobe, one in my handbag, one in my kitchen 'everything useful' drawer and one in the keys drawer in the hall. . I also have sharpie marker pens in all those places (except the dc's wardrobes) in case I need to mark non-clothes things.
I buy a packet of cheap envelopes to send money or stuff back into school in. In a quiet moment I'll mark a few of them with dc's name and class, plus a few more will have name, class and lunch money written on them so that when I need one in a mad hurry after sleeping through the alarm clock again I can grab it and it will be ready. I also try to keep a biro ready with them so there is always one when needed, especially useful if filling in forms to be sent back to school.