What are kennings for police, boyfriend, and prom?
Keep in mind that a kenning is a literary trope, or rhetorical figure of speech, which typically combines two words, usually hyphenates them, and uses them figuratively to replace a one word noun. Kennings are most common to old Islandic and Anglo-Saxon poetry.
Kennings can be humorous or not and are very similar to puns in that they play on common words using common words to describe something in a new and different way. To remind you of a few examples the sun has been written in kennings as: sky's-jewel and sky-candle. The author of Beowulf described the sea as a "whale-road."
I'm going to guess that you've been given a creative assignment here to create your own kennings for the words above (afterall, "police" and "prom" are far too modern to have likely been used in a piece of literature employing kennings). While I could personally provide several ideas for you, I think you would have more fun coming up with the kennings yourself. Another point to add is that the words you choose in the kenning is going to reflect your attitude about the original word. An example for this might be the kenning you design for boyfriend. If you wish to portray a committed relationship (in highschool) as a positive or whimsical thing you could change boyfriend to "phone-ringer" implying that he's always calling. However, if this is a negative thing, the same role could be called a "prison-guard." Do you see where you could actually have a lot of fun with this?
Also, if the kennings themselves are going to be used in a poem (written by you), it is not uncommon to create a figure of speech for the purpose of matching a rhyme. Shakespeare did this all the time.