Hi Landon!
Thank you for bringing your question to the Disney Parks Moms Panel!
As a general rule, when school is in session, the crowds typically are lower. September is usually a very low-crowd level month as schools are starting back and it's also still very hot in Orlando. I homeschooled my boys for ten years and we often traveled to
Walt Disney World Resort in September because I knew the crowds would be very low! Labor Day weekend tends to be more crowded than the rest of the month since it is a three-day weekend for most jobs and schools.
If you Google "Disney World Crowd Calendar", you will find some online resources with crowd prediction calendars. They are usually based on previous year's crowd levels and information they have about special events, discounts, etc. While no one can completely predict how crowded a park will be, these do give you some good guidelines.
If you do decide to go in September, be sure to check out the
Epcot International Food and Wine Festival. It begins on August 30 and lasts through November 12. The festival is less crowded on weekdays than weekends.
I hope this helps! Please let me know if you have any other questions as you plan your vacation.