tex-first-line-header-regexp

tex-first-line-header-regexp is a variable defined in `tex-mode.el'.
Its value is

nil

Documentation:
Regexp for matching a first line which `tex-region' should include.
If this is non-nil, it should be a regular expression string;
if it matches the first line of the file,
`tex-region' always includes the first line in the TeX run.

You can customize this variable.