This line sets us up and gives us the context of the poem. There's an instructor, so we can guess that we're in a classroom. Notice how this line is set apart from the poem? Since the instructor gets this first, separate line—at the very top of the whole poem—we get the sense that he's an authority figure, and we're going to listen up to what he's saying.
Go home and write a page tonight.
These lines are in italics, so after the set-up of line 1, we know that this is probably the instructor speaking (italics in writing tend to indicate someone's speech or thoughts). He's giving out his assignment. His students are supposed to write one page. (Terminology note: Back in the day, such writing assignments were called "themes." So, "Theme for English B" refers to this assignment, not the English "theme," the central message of a literary work, that we typically think of today. For more on this, check out "What's Up With the Title?".)
So, write about what?
Sounds simple enough to us, but it's also pretty vague. And it makes us think about how different home could be for each of this instructor's students. Where do they live? Who do they live with? What is their commute to class like?
And let that page come out of you— Then, it will be true.
Here, we get deeper into the assignment. This isn't just any page, but a page that's supposed to come out of "you," or, the student. This means that whatever the page is, it's supposed to reflect something deep about who the writer is. The topic seems to be wide open, though. Whether it's their sentiments about scrambled eggs or the state of modern society, if it comes from the inside, it's fair game for this assignment.
Line 5, then, goes a little further to say that, if the page comes out of the person writing it, it will be "true." The professor is inviting the students to see the connection between their writing, their selves, and some ultimate truth. Hmm. We wonder where that will take our speaker.