Tense Shifts & writing about autobiographic pieces...

Hi everyone...

I am having students (12th grade honors) compare Mary
Rowlandson's "Narrative..." to an excerpt of Martha Martin
(stuck in an avalanche for four months, etc.). Students
submitted papers today, and the tense shifts are driving
me crazy!

Here is my question: If students are comparing parts of
the works (which are autobio's), I think they should write
in present tense. They argue that b/c they are speaking
of the authors and the events they encountered, past tense
is appropriate.

Any feeling one way or the other? Is it okay to switch
from past tense in one paragraph that introduces the
author experiences to present tense while discussing the
work?

I'm confused! Thanks!