Why do the girls keep laughing at word like "skull" during the field trip? In the episode 4 of the second season they keep laughing randomly when words like "skull" etc are uttered. I don't understand the rationale behind it.

Note- I am watching Japanese audio, English subs.
I'm Japanese but it's incomprehensible little bit to me too.
髑髏 (しゃれこうべ) is not usual word in these days and sounds little bit strange (goofy, should I say?). We use 頭蓋骨（ずがいこつ）to mean "skull".

It's just a joke without any reason. but somehow it's funny in certain situation, like tense situation or lifted up at midnight on trip. Because, there's no reason.