Good subject, tough issue, local answers.
I've been in churches and school districts where the public schools went back in early August, or late August, or after Labor Day. In most of those churches, we followed the public school calendar.
In the U.S., the kicker is our Labor Day holiday which occurs the first Monday of every September. It creates a 3-day weekend right in the middle of any August momentum you have built.
I've done Sunday School in a lake community where the official end of the season was Labor Day weekend. Whether our people were actually AT the lake or not, it created a psychological barrier to starting in August, but it also created a "back to normal" impetus right after the season "closed."
In general, I'd rather start early to mid August than late August right before we had a three day weekend.
I also think the answer to "when to start" depends on how much of the summer your program(s) "took off." If your Sunday School and children's ministry have taken all or most of the summer off, then getting a late start would eat into even more available Sundays, and we don't want to give up even more Sundays!
A corollary to this issue is when to "end" regular Sunday School in the Spring. And again, I've been in churches that "ended" regular program on Memorial Day or the first week of June, and those that went through June because the local schools were still in session.
Of course, kids don't drive themselves to church, so unless you have childcare and adult offerings, it seems like a fool's errand to try and restart Sunday School before these other adult offerings are available (or end in the Spring before adult classes do). For that reason, I like Carol's suggestion of "intergenerational" classes so that the parents can participate if the adult ed program hasn't started up yet.
I like and have used the idea of a "soft start" or "special start" that expects fewer students and offers a different kind of Sunday School in the first week or two. For example, in a former Florida church, we planned outdoor Bible lessons on late August Sundays with activities suited to the weather. That church had adult ed at the same time.
Rather than putting all our eggs into the fall startup basket, or fighting traditions and ingrained attitudes and schedules, we might be better off looking at some of the other days and times of the year when we can RECOUP lost teaching time. I've written an article about "how to find more time for Sunday School" here at Rotation.org with lots of examples.