For the first letter of The Penguins of Doom, I would put "Pompeii" by Bastille in the soundtrack. Every time I hear it, I have to wonder what the lead singer did to make his backup guys call him an "a-hole" over and over again all through the song, just like I'm wondering what the assistant principal did to make Septina write that letter to him.
For the interlude that follows (not published to Tapastic yet, but soon), I have had "Burn" by Ellie Goulding in my head. You'll see why. Cuz we're gonna let it burn, burn, burn, burn...
For the second letter, you'll want to be listening to "Can't Buy Me Love" by the Beatles.