How do you make sure an expository essay doesn't become a persuasive one?It seems just like a persuasive essay because my teacher said you can pick...

No doubt that you have stumbled upon some of basic element
of writing.  The concept of "good writing" blurs the distinctions between narrative,
expository, and persuasive. Naturally, there are some attributes of each that are
present.  Few would deny that.  However, good writing that tries to convince a reader of
an authentic voice is going to be persuasive, even in an expository format.  Good
writing, then, is something that stands on its own.

I do
think that you might need to take some steps to make sure that you are giving what your
teacher wants.  In the end, they are grading and assessing your work and it is important
to make sure you put forth your best effort in this realm.  One distinct undertaking is
to make sure that your expository essay involves a reliance on facts and analysis.  It
might help if you sought to eliminate the use of "I" and personalizing the expository
essay.  In a persuasive, you are able to appeal to your own sensibilities in trying to
persuade your audience.  In an expository piece, you want to stick to the facts and
stick the analysis you are trying to present.   For example, if you are to write about a
work of literature, your expository piece would highlight scenes, lines,
characterizations and you would be sticking to these.  In a persuasive piece, you might
be permitted to add in your own analysis to these, something that might not be as openly
embraced in the expository piece.  In the expository piece, your thesis statement is
probably going to be centered on the work, in general.  In a persuasive piece, your
thesis is going to be based on  point of view of which would like to convince the
reader.  Keeping some of these elements in mind might help you keep your piece strictly
in one domain.

The final solution here would be to offer up
drafts of your work to your teacher.  This is going to be dependent on what the teacher
says, but I cannot imagine anything wrong with you asking the teacher to review some
draft of your work to make sure you are on the right track in terms of expository vs.
persuasive.  It's a good point you make in that the line between them can be blurred
easily.  Good writing tends to do that.  Asking your teacher to help clarify will help
you be clear and probably help out your grade.