I'm a special needs teacher myself, and I would never tell a family not to go on vacation, but I would request that the student take some work with them. If your child's teacher doesn't offer any, my advice would be to ask for some practice work, and make sure it gets done so your child stays within the swing of things!
Last time I had a 6th grade student go to WDW on vacation, I made up a few fun assignments that could be done in the parks (like looking at the headstones at the Haunted Mansion and then creating one about somebody you know). I had a great time creating them, but when my student returned, he had completed NONE of it! I was really disappointed and upset that the assignments (and they weren't many - just enough to keep my student thinking!) weren't taken seriously. So have a great time, but make sure you keep some sort of routine as far as getting any assigned work completed. You'll be glad you did when your child goes back to school and doesn't have a difficult time transitioning back to the regular schedule.