This article is from the Financial Aid, Scholarships, and Fellowships FAQ, by Mark Kantrowitz with numerous contributions by others.
Many parents choose to file the 1040 even though they are eligible
to file the 1040A or 1040EZ. Likewise, companies like H&R Block
often automatically file the 1040 form, regardless of the AGI or
earnings. This can sometimes cause their children to not qualify
for the Pell Grant.
Parents should ask their tax preparers whether they would have
been eligible to file a 1040A or 1040EZ. The question is NOT
whether you would have gotten a bigger refund with the 1040, just
whether you would have been eligible.