Dennis Baron (born May 9, 1944) is a professor of English and linguistics at the University of Illinois at Urbana-Champaign. His research focuses on the technologies of communication; language legislation and linguistic rights; language reform; gender issues in language; language standards and minority languages and dialects; English usage; and the history and present state of the English language.
From Wikipedia
“Yes, teachers are expected to model good grammar as well as teach it—they could lose their jobs if they don't—but no matter what you’ve been told before, singular 'they' is grammatically correct, and the American Psychological Association's influential 'Publication Manual (7e)' is the latest authority to agree.”