I have had 4 stepsons in my home at various times. They were 10, 10, 12 and 15 when I married their dad. They are now 20, 18, 15 and 15. Only one is still living with us.
They all went through a few "school/behaviour" phases. I will tell you what worked for us:
1. Kids over ten should be responsible for keeping up with their clothes and school supplies. If they are late because they can't find their shoes, then they get a tardy. If they can't turn in their homework because they didn't repack their school bag the night before, they get the bad grade (no sympathy from us for being lazy) ... you must let them face the consequences of their actions or they will never learn to be responsible.
2. Decide on punishments for failures to keep up with school work. Write it all out. Tell them what you expect from them. Stick to the punishments. We have an assignment book. They are expected to manage their school work. They have to write down what they did that day, what assignments they were given and when it is due, what grades they recieved on tests, quizzes, or homework, and when the next test will be. If they don't keep up with this, they are grounded for a night or until they catch up... If we find out that they lied about something having to do with class, they are grounded from the computer for a week.
3. Don't cut them too much slack... they will just take advantage. If they need help and reassurance, then they need firm guidance and boundaries/rules to rely on also. Expect more from them, not less. Spend more time with them, not less. Don't just ground them to their room and think they learned a lesson. Be creative and interactive. Use restrictions and rewards. If they spend too much time playing a computer game and it interferes with their homework and chores, then set a time limit on that game. They will never limit themselves... you have to do it for them.
4. Talk to them about what they did wrong and what they should have done until you are blue in the face. We adults tend to think they understand, but pre-teens really are aliens (their brains are actually changing dramatically), but they don't understand... the only way they will is if you tell them exactly what you think and then ask them what they heard you say until you are both clear on the issue.
5. It never ends... they will do something the same way for 6 months and then suddenly forget. They will swear, when you bring it up 2 weeks later that they don't remember ever doing it (like vacumming the hallway). Are they lying? Are they braindead? I don't know. The action we take as parents is the same. Show them that list of expectations/rules/chores you should have written up and posted in the kitchen and tell them to do it and remember next time.
6. If you don't write it down, they will always be able to say that you "never said that." Write it down! I don't care how wonderful they were before they were ten. They will never return to that child. They are going to continue to grow and become more and more alien, rebellious, and brain dead. How do they go from a child who got up at 5 am to watch cartoons to a teenager that won't get up until noon?! It will happen! Pick your rules and stick to them... if you don't do it, no one will. It will give you all something to cling to when things get really confusing.