Cassandra's question "who protects the Guardian?" is essentially Juvenal's famous "sed quis custodiet ipsos custodes?" (But who will watch the watchmen?") and philosophically traces back further to the Platonic notion of a guardian (Gk. phylakes) class as an essential of civilized society.
When Jacob says, "So, your explanation for impossibly high grades is you 'O Captain! My Captain!' them?" it is referring less to the original Walt Whitman poem, and more to its use by John Keating (Robin Williams) in the 1989 film "Dead Poets Society", where the inspirational teacher changes the lives of his students.
The Reaper is played by British-born stunt performer Buster Reeves, the man who was Christian Bale's double on "The Dark Knight" (2008).