Schoolgirl chronicles a day in a life of a Japanese girl. It's a first person narration full of angst, snark and minute observations. From a casual mention of a war, it's fair to assume that the story took place in the early 1940s, but it really doesn't matter, because she could well be just another teenager from our time, from any country. The narrator has an acute self-consciousness not uncommon in teens and a degree of self-awareness that is more typical in someone older. And in a true stream-of-consciousness fashion, you'd get to see her contradict herself without any sense of irony, from badmouthing her mother to berating herself for being such a bitch to wishing that she could be a better, nicer girl.
For an introduction to Dazai, I think Schoolgirl is as good as any. It's short, and it gives you a good idea about his prose and the feel of his works. For me, it's just an OK book, but I suspect that's because I've read No Longer Human, which, some people believe, is his best work. But I'd recommend Schoolgirl to anyone, especially if you're looking for a quick read, and if you like angsty/moany youth confessionals like The Catcher in the Rye and Botchan.