everyone cries at leavers assembly. Everyone.
OP, if your DC is anything like me ds1 then
He will need an addidas messenger bag. Despite the fact nothing fits in it. Barely books and a pencil case. He has a seperate bag for football boots, PE kit, cooking stuff, folders etc.
Velcro shoes are a no. But kickers are a must.
PE kit comes home after every lesson because for 3 years he has been too late to get a locker because he forgot to hand in the form.
He will never wear a coat. In freezing temperatures he will wear football skins and a hoodie. He will then try and cram the hoodie into the above mentioned useless bag.
He will lose the Oyster card on average once a term. Until you tell him that the next time he loses it, he will have to pay the £10 to replace it out of his own money. Or walk to school. At that point the Oyster card will always be in a safe place.
I have 2 DCs now in year 10 and 9.
The best advice I can give?
Make sure they are confident of the route. And that they know how to get there if the bus/train in cancelled.
Make sure they have a bit of emergency money just in case.
They will need to do a lot of homework using the computer. They will well mine will think they can do this homework in between FB/twitter conversations and updates, listening to music and watching films on you tube. So half an hour homework could turn into 4 hours uninterrupted Internet surfing if you don't monitor it.
They will change a lot in a short space of time. They will become more independent, more "grown up" Be prepared for that.
They will still need help at the start. Help with organizing their time, homework schedules, reminding then to get PE kit ready, pack bag the night before and all that stuff. I did this for the first half term. I warned them this was a short term thing and I wasn't going to be doing it forever.
They will have friends that you don't know. They will want to go places that they have never been. They will go to friends houses and you will think "but I don't know the parents. I don't know what type of family they are. What if they are [insert whatever horrible thing you like here]
But.....you can't stop them growing up. You have to, for the most part, hope you have raised them in the right way and have faith they won't do anything ridiculous. But....sleepovers and parties? always verify that with the host parent 
They will eat ridiculous things for lunch. Our school has a place online where you can check what they have eaten. I stopped checking ds1 when it said "coke. Double chocolate muffin. Hot chocolate" 8 days in a row. 
You may want them to eat a decent lunch. But sometimes you have to pick your battles.
Wow that was a mammoth post 
I hope it might have been helpful to someone at least.