Joss Whedon is all about parallels in plenty of his work, especially his television series. In this scene, where Xander enters his class late and is called out by the teacher is a thematic parallel to the final battle that will take place in Pt. 2. Xander’s representative of multiple players within the battle. The first, being that of the vampires (or even Angel): the late comers to the party are still expected to help and attain the final goal. The class’s goal is to determine the word chosen by the teacher in hangman, similar to the events leading up to the final battle… The scooby gang must determine what the Mayor is planning and how to fight against it. Xander’s choice of the letter e is a bit more abstract but can be seen as representing those who flee (or exit the ceremony) when the battle begins. They’re attacked and potentially are killed. The teacher laughs at Xander’s choice with one of those cartoonesque evil laughs mumbling “they always go for the e,” similar to how the Mayor knows they’ll be some that attempt to exit the ceremony immediately during his ascension. As a trait of Whedon, he is a master of foreshadowing in unexpected ways (Dawn is coming… dream sequences, etc.), but if you look back at his work after knowing what is going to happen, it is nearly impossible to deny the facts of what is seen before hand that leads up to an eventual event.