The person who diagnosed your child with ADHD has probably explained to you already all the reasons for the difficulties. If not, let me refer you to the following books: Attention Deficit Hyperactivity Disorder: A Handbook for Diagnosis and Treatment by Russell Barkley, Driven to Distraction by Ed Hallowell and John Ratey, and It's Nobody's Fault: New Hope and Help for Difficult Children by Harold Koplewicz. Each book provides at least a chapter on the medical bases for the symptoms of ADHD. The National Research Center on the Gifted and Talented also carries some outstanding monographs that summarize research on the topic.
As to your questions about what you can expect from the schools and where to get help, let me start by asking you to take a few preliminary steps: Is your son forgetful primarily in one class or is it in all his classes? In some types of assignments but not others? Does the problem appear to be random or is it consistent? What does the problem "look" like? Does he leave materials at school when they are supposed to be brought home, or does he leave things at home that are supposed to be brought to school? Is he usually late? In other words, try to look at the problems as specifically as you can.
Another way of thinking about disorganization and forgetfulness is that these are issues related to inefficiency, not inability. And if that is true, you need to know the tasks in which he is particularly inefficient. From there you can go on to remedy the problem in concrete and logical ways. For example, if the problem is that he always forgets take-home papers at school, you can give him a three-hole punch to keep at his desk. That way, any paper that crosses his hands gets immediately punched and put into a notebook that he can sort out later. What needs to happen is that your son must learn to create new habits. Before he can do that, however, he needs to know which behaviors and situations he should focus on. You need to be as creative as possible in establishing new habits -- as you know, what may work for students who do not have ADHD might not necessarily be effective for those who do. So try to think broadly about the solutions, even if some of your ideas are unconventional. Remember to get your son's assistance in this. He may himself have some intuitions about possible solutions, even if he can't think of them at the time he's confronted with the challenging situation!
Many books about ADHD have excellent strategies that target specific behaviors -- there are so many on the market that I hesitate to recommend just one or two. A trip to a nearby bookstore or library should give you some good ideas. For accounts of real-life experience with real kids, you might also want to check the following websites:
Our own LD/ADHD homepage,
These sites have information and resources for and by parents who have faced the kinds of issues you are asking about and some of their solutions might work for you.
As for the school's responsibility in assisting you with these problems, you might want to ask the school counselor or principal about your son's 504 plan, which outlines the accommodations to which students are legally entitled. Accommodations such as extended time may help your son with some of his disorganization problems (Remember, if this is a problem of inefficiency, more time to do his work might help). Or you might want to consult the previously mentioned websites to find out more about your son's rights, as well as the school's responsibilities. In addition, such organizations as CHADD (Children and Adults with Attention Deficit/Hyperactivity Disorder) would be able to give you information about people in your area who might be of help, including the names of some professionals who could provide extra tutoring in organization and time management if the school is not able to offer sufficient accommodations. Incidentally, the annual CHADD conference always includes a great vendor area that has all kinds of products to assist students with disorganization and forgetfulness! Good luck!