"Four O'Clock Friday" is a poem written by John Foster in which the first-person narrator tells us about their unhappy life at school. We can tell that the speaker is meant to be a child through lines which talk about the bullying the narrator experiences at recess. Therefore, the main theme of this poem is clearly bullying, as the speaker tells us that at school his peers do something to him to upset him every day. This is why he looks forward to Friday afternoons, as this is the time he gets to go home and he is able to start the weekend away from the bullies. This theme is underlined by the choice of title, as "Four O'Clock Friday" is when the speaker "is home at last."
In terms of poetic devices, you could mention the fact that the poem is written in rhyming couplets. Each couplet deals with a day at school, so by using these rhyming couplets, the poet puts even more emphasis on the fact that every day at school is a nightmare for the speaker. You could also mention that the title of the poem contains an alliteration: the letter "f" being used twice in the title stresses how important this time and day are for the speaker. The speaker has been looking forward to "Four O'Clock Friday" all week, as this is when his suffering finally comes to an end.