"Half-Past Two" begins as if the speaker were about to recount a fairy tale, riffing on the cliché opening "Once upon a time." The poem takes place more specifically "One upon a schooltime"—a compound word that establishes the setting—a classroom—while also foreshadowing the way the poem's main character conceives of time.
Line 2 then introduces that main character: a young boy, who is referred to only as "he" and "him." The speaker declines to give him a name just as, later, they will refrain from naming his teacher. This anonymity makes the poem more fablelike; he could be any boy, and this could be any classroom.
The speaker informs the reader, in no uncertain terms, that the boy has done "Something Very Wrong." Readers never learn exactly what that "Something" was. Still, the phrase "Something Very Wrong" is capitalized for emphasis—clearly, this is serious stuff; the phrase might be ambiguous, but the capitalized words are formidable and imposing. They communicate the angry tone of an adult chastising a small child.
And yet, the details of this incident are left intentionally vague throughout the poem, as if the episode were only half-remembered. In fact, the speaker admits in the next line that they "forget what it was" that the boy had, thoroughly undermining the idea that something truly heinous has taken place. If the speaker can't remember what the boy did, his terrible crime couldn't have been all that serious after all!
The first stanza of the poem is thus effectively a bait-and-switch. It sets the reader up to expect one thing, but it then delivers the speaker's parenthetical like a punchline. This goes a long way in establishing the poem's wry tone. Although the speaker takes the young boy's feelings seriously, they also put the whole episode into perspective.
Metrically, the first two lines play another trick on the reader as well. They set the reader up to expect a somewhat regular pattern of stressed and unstressed syllables:
Once upon a schooltime
He did Something Very Wrong
The trochaic meter of these first two lines has a childish, sing-song quality to it. It actually sounds quite a lot like a nursery rhyme. Line 3, however, breaks the pattern. Like line 1, line three has six syllables; unlike line 1, those syllables can be broken into two anapests instead of three iambs:
I forget what it was
Because anapests are made up of three beats, not two, one might say that an anapestic meter is a little more complex, or even grownup. The switch in meter coincides with a sudden shift in tone—and with the adult speaker interrupting the narrative to provide some grown-up context.