In general, when do schools start classes?

Every state has a required minimum number of days for the length of the school year. H E R E is a list of all the states and the days required. Shortest is 160 days; most states, it's 180 days. Add in the holidays at Thanksgiving and Christmas, as well as spring break, and in-service teacher training days, and you can generally figure that schools will likely begin around Labor Day, and end some time around Memorial Day...plus/minus a few depending on states with snow days...or rain days...or hurricane days....(or when I was growing up in Alabama in the 50s-60s, "cotton picking days" in the fall)

My daughter is starting her student teaching this year and is already back (in Colorado) for in-service training. Most, but not all, school districts in Colorado will be starting up over the next 2 weeks or so.