Pencils. Why pencils? - Another question for Americans

[thismaz]

I was watching BtVS3:1 'Anne', the other day and there is a scene with Willow and Xander in the library, when Giles rushes off after another possible lead on Buffy's whereabouts. They are studying and both of them are holding pencils. These two are supposed to be in their final year of High School. They are 17 going on 18. Why don't they have pens? And it occurred to me, again, that I can't remember ever seeing any of the High School students use pens. It is always pencils.

So I'm asking if there is a reason for that. Anybody know?