Teach students the definition of personification, then supply them with examples of poetry that exemplifies good use of the technique. You can find these in the classroom literature textbook. Even though seventh graders are young, they should be exposed to poetry that adults have written for adults. You can pick pieces like Emily Dickinson's "The Sky is Low" or Langston's Hughes' "April Rain Song." Or you can use an anthology like "Learn Then Burn" edited by Tim Stafford and Derrick Brown, which contains classroom-appropriate poetry by modern poets.
Point out the places in the poem where personification occurs and ask students to comment on the use. Have them tell you what they believed the use of personification accomplished for the poem, and why the author chose to use it. Have them write a few sentences that they can share in class or turn in to you.
Ask students to choose an object or animal that they would like to personify. Have them write a poem from that item or animal's point of view. This will give them a broad overview of the technique of personification. A good example is this excerpt from Sylvia Plath's poem "The Mirror," which reads, "I am silver and exact. I have no preconceptions./Whatever I see, I swallow immediately./Just as it is, unmisted by love or dislike/I am not cruel, only truthful." Collect the poetry and review it.
Ask students to write another poem that is not specifically a personification poem. After they have written it, ask them to pick one item mentioned in the poem. Have them rewrite the poem but give the object a human trait when it gets mentioned in the poem. For example, they could create a line like this one from "Root Cellar" by Theodore Roethke: "Even the dirt kept breathing a small breath." This gives students experience using the technique within the poem.