Will a conference paper submission have greater credibility if a teacher is a co-author?
Authorship should not and, apart from inherent human bias, is not taken into consideration when evaluating a manuscript for publication. If the conference is reputable, you should trust that the committee will attempt to minimize this kind of bias by choosing equally reputable reviewers.

Rather, what you should be pondering is whether your department head has contributed sufficiently to the proccess for them to be granted authorship. Conventions vary by field: see this and this.