The poem never says the word "school," but the clues are easy to follow. Children pour out of "Jails" when it is "Afternoon," and the place that holds them all day and lets them go at a fixed hour sounds very like a classroom. That is an inference, but a safe one. The central idea is that children's joy is a physical force, too big for the buildings built to contain it, and that it will soon meet the disapproval of adults. Dickinson gives us the burst of freedom and the shadow behind it in just eight lines.
The first stanza turns on its comparison. Calling school a jail is a joke, but the verbs make it feel real: nobody walks out of a jail, they "leap." The adverb "Ecstatically" is the longest word in the poem and it sprawls across the line, as though the word itself has to run to keep up with the children. Then comes the strange third line: "Beloved only Afternoon / That Prison doesn't keep." The grammar can be read two ways. Afternoon is the one beloved thing the Prison cannot hold, or it is the only part of the day anyone loves. Either way, the Prison has the rest of the day, and Afternoon is a kind of prisoner who escapes with the children. It also has an extra syllable compared with the lines around it, which gives the stanza a slight stumble, like someone hurrying through a door.
In the second stanza the images turn military. The children "storm the Earth and stun the Air," and the two verbs lock together through alliteration, so you can almost hear the noise: shouting, shoes on gravel, a door banging. Then comes the best phrase in the poem, "A Mob of solid Bliss." "Mob" usually suggests anger or danger, and "Bliss" is the opposite, so the two collide. "Solid" makes an abstract feeling into something with weight, a crowd you could bump into. This is happiness as a physical mass, and the word choice makes the children both comic and a little frightening.
The last two lines bring in the adult world. "Alas" is mock-solemn, the sigh of someone who understands exactly what is coming. The "Frowns" are a clever stand-in for authority: Dickinson does not name teachers or parents, only the expression on their faces, and the capital F turns them into soldiers who "lie in wait" in ambush. The twist is the word "Foe." We expect the children to be the victims, but here they are the dangerous side, a Foe the Frowns must face. Whether the "Alas" pities the children or the Frowns is left open, and I think the poem is happy to keep both readings. The final dash gives no result. We never see the collision, only the moment before it, with all that energy still running.
The form supports the contrast. The poem is built from alternating longer and shorter lines, close to the common measure used in church hymns. The second stanza follows it exactly, with eight syllables, six, eight, six. The rhymes "leap/keep" and "Bliss/this" fall on the shorter lines, like a small gate clicking shut at the end of each stanza. It is a nice irony that a form associated with solemn worship is used for a playground stampede. Dickinson's habits of capitalizing nouns (Jails, Boys, Girls, Prison, Earth, Air) and using dashes are all here, and the capitals make ordinary things feel like characters in a little drama.
What makes the poem worth reading is how much it does with so little. Dickinson lived in Amherst, Massachusetts, and most of her poems were published only after her death, so she wrote with no audience of schoolchildren in mind. Yet she gets the feeling of the last bell more accurately than many longer pieces of writing. She takes the child's side without turning children into sweet little angels, because her children are loud, overpowering and almost dangerous. The poem also hints at a larger idea that applies far beyond school: freedom is most vivid when it is brief, and the "Frowns" are always waiting just beyond it. Anyone who has run out of a building at the end of a long day will recognize the moment.