Our school uniform is a great shade of what I think is Cobalt Blue. It is the first thing I thought of, so I had my students gather together in a globby-mob and I snapped a few shots. There might be a bit more depth here about uniformity, privacy, (no heads in images) etc...but I will let you sort that out.
Today's theme is blue. Make a photograph dominated by the color or that otherwise illustrates the theme.