Here is how we have handled it in the past, and again this year...
Our school sends home a guide book at the begining of each school year, outlining all of their policies and such. Inside is a form used to request advance permission for an 'excused absense'. Generally, anything that is considered an educational experience & that serves as an appropriate supplement to classroom learning--trips to WDW included--may be excused. So we filled out his request form and submitted it on the second day of school. Maximum number of days a student may be 'excused' from classes (in our district) per year is five--anything over that is considered 'unexcused', and students are not permitted to make up work that is missed due to an unexcused absence.
My DS had actually already told his teachers on the first day that he would be traveling to WDW in October, so they are aware. Our school holds a 'Back to School Night' next week, where we have the opportunity (as parents) to meet with his teachers in person... and we will be talking with them all about the upcoming trip to FL then as well. When we get closer to the actual dates for our trip, I will likely follow up by sending written notes in with my son for each teacher, requesting their guidance as to what work he may be able to complete in advance or while on our trip, along with asking if there are any special assignments they would like him to complete relative to his WDW experiences.
Two years ago, his teacher simply asked that he keep a written journal of his trip, in which he recorded what interesting facts he learned each day and included photos that he took himself. He also completed an EPCOT passport (in which he had CM's from each of the countries write a little something about their homeland in their native language), and then he later gave a presentation in front of the class regarding some of the things that he learned & experienced. The teacher(s) also began giving him his 'regular' assigments & homework sheets about two weeks in advance of our trip, so that he could complete much of his work prior to (as oppossed to during) our actual travels. DS not only had no trouble at all keeping up with his classwork, but actually was about a week or two ahead in his lessons when we returned (in comparison to the kids who had been in class). He has always been a solid 'A' student... so I do not have many concerns about his missing a few days of classess this early into the school year.
My advice would be to check into what your own school district's policies are regarding absenses due to travel. And then also take the time to talk with each teacher in person, so that there are no unpleasant surprises.
