"at" at the end of a where phrase

[Jan Kammert]

> I too had a teacher who taught us how to analyze Latin-based words (a
> useful tool I wish my grad students had), do library research and write
> decent papers (another skill often lacking), and appreciate good
> literature.  But when she tore down a sign at a bake sale during a
> basketball game because it said "Boughten Cookies," she shamed us and our
> origins.  It was years later, of course, that I learned 'boughten' is a
> common Northern participial adjective and felt vindicated.
>
I teach 8th grade.  This thread makes me feel as if I should never correct
my students writing.  No doubt, I will think something is wrong that
really isn't, or is so common that I should just ignore it.  Where is
there help for teachers who were never trained about language themselves?
Jan