T.A.
My daughters both wore short sets the first day of school. One is in first grade and the other is in third. It is still summer and the weather is still warm.
I see no problem with kids wearing comfortable clothes to school even on the first day. We do not know their schedules and they may have gym that day. If they dress up, they won't be comfortable in gym. That is my opinion. I hope it helps.
T.