Tone is generally defined as how the author feels about his or her subject. You have been asked to infer what Gwendolyn Brooks was thinking and/or feeling by writing "We Real Cool."
The first thing we note is the non-grammatical title: "we real cool," which lacks a verb. This does have the sound of inner-city chat or slang...like verbal shorthand. This may infer that speaker (and this is not the author) is not educated...but they think they are cool.
What do we think? Can you be cool if you don't speak well? Well, maybe. We read on, perhaps not sure.
The person speaking for "we" notes that they have quit school: this makes them cool, or so they think. The reader may wonder: how will you find a job? There's more. Maybe we can better understand if we keep reading. They "lurk late" which means they hang out late at night. Well, that's not necessarily a bad thing, is it?
Ahh, but they "strike straight!" This is something very different, and this changes the mood instantly with the word "strike." I can only imagine this has to do with a stabbing. There is nothing that directly identifies a knife; if the poem simply noted "striking" by itself, it could refer to landing a punch, however, the fact that it is straight indicates to me that a sharp weapon is in hand, for using it in a straight motion would make it more effective than a glancing blow.
The next couplet gives the audience relief. They "sing sin," so they must sing songs about doing bad things. I can imagine many people thinking of the lyrics of bands—past and present—that sang the same kinds of songs, without blinking an eye: they are just words...and singing can't hurt anybody! Then they "thin gin." They drink liquor, but water it down. Maybe they won't get drunk so fast. Hmm. Maybe they are cool!?
They then "Jazz soon." The speaker has already spoken of singing, so this must mean something else. The informal use of "jazz," as a verb (or an action), is defined by Dictionary.com as:
a. to excite or enliven; b. to accelerate.
So the speaker's friends must liven up the month of June...the beginning of summer—when the days are longer, the windows are open and people visit over fences or across the stoops (front steps) of buildings. There are a lot of things to do in June—it is a month people look forward to: school is out (for others) and vacations are anticipated. It is, however, the last sentence that puts everything into perspective. In light of all that "we" do in this poem, which is supposed to convince the reader that "we be cool," the poem ends with:
Except for the "strike straight" (which we may not have believed we really understood), we may be startled to learn that the behavior of these young people leads to their deaths. For "die soon" may mean "imminently," but it also may mean "too soon"—a phrase our culture associates with young death—death before "one's time."
Brooks is saying that these kids think they are cool, with their "street" talk, quitting school, drinking, hanging out, and singing about things they should not do. "Strike" introduces a sense of violence, hostility. And death soon follows: are they killed by someone else with a knife, or by the police? Brooks' tone is that if death is the result, these kids are certainly not cool. They are kidding themselves; and she doesn't believe it, nor should we, not for a minute.
This is a powerful poem. Isn't it amazing that Brooks, in so few lines, is able to convey her very strong message!