Likely a reference to the war survival movies of the 50s. Students
were instructed to duck and cover, kneel on
the ground with their hands over their heads, in the event of an bombing.
In retrospect, it seems like a rather unlikely way to survive a nuclear
blast.
Given that the show is set in California, this could also be a reference to a school taught earthquake drill.
Xander: Look, I don't mean to poop the
party here ...
A play on the expression party pooper.
A party pooper is a person who ruins everyone's fun at a party by not having
fun themselves. It's grown to mean anyone who squelches another person's
enthusiasm.
Buffy: We've gone public.
Normally a person goes public, making
something formerly a secret generally known, willingly. Here Buffy uses
the expression to refer to the fact that their rebellion is know widely
known.
Buffy: Wanna see my impression of Gandhi?
Mohandas Gandhi was a revolutionary leader
in promoting religious tolerance and self-rule in India. While he may have
be "really pissed off" he is most known for his non-violent approach.