These words conjure up a vivid image of the Venusian children pressing close to each other as they look out of the window for any signs of sun. Bradbury's description of the children as "roses" highlights their innocence, while his likening them to "weeds" emphasizes their wild, unruly nature. Like most classes this one's clearly a mixed bag. But all the children, roses and weeds alike, have one thing in common: they're dying to see the sun.
None of the children, with the exception of Margot, have ever seen the sun, as it only comes around once every seven years, and so no one else in class is old enough to have seen it. Therefore, it's not surprising that there's so much anticipation in the air. The imminent appearance of the sun is eagerly anticipated, not least because it'll provide brief respite from the constant rainfall.