The kids at Rock Creek Elementary School have never been much for scruffy jeans or an unkempt look, according to the shcool's principal, Louise Rosenberg. "But opening day is always a little special, like dressing up for a field trip."

So in spite of yesterday's 90-degree weather, specially bought school clothes for fall dotted the school playground: pinafore dresses, T-shirt dresses, and T-shirts or blouses with skirts.

Hemlines tend to be a bit longer than last season, partly the result of the fashion for longer skirts for little girls and partly practical mothers' desire to leave skirts long enough to grow into.

For boys, dressing up is less apparent. Rugby-style knit shirts and safari shorts with lots of pockets are very popular.

For most of the children, putting on their best duds was in response to "mom's orders," as Molly Horton, 7, put it. Molly, who was wearing a cotton skirt and blouse, said he was tired of pants for the moment, but she and her friends expected to be back in them before very long.

Said Kathy Gendel, 8, "Skirts are all right when you are going to have your picture taken. But for everyday," she said, encouraged to do a somesault mid-sentence, "nothing beats pants."