While some teacher lies may be due to ignorance, the cold hard truth is that most are lies of convenience. It's much easier to tell students you can't begin a sentence with "because," "and," or "but" than it is to fully explain the workings of subordinating conjunctions and coordinating conjunctions.
Throughout the school year other lies told by beloved previous teachers are put to the test as well. All are well-intentioned and undeniably useful. Not coincidentally, they're all designed to distill writing down to a convenient formula. "A sentence has four to ten words." "A paragraph must have five or six sentences." "A good essay must have five paragraphs." Good writing can never be achieved by focusing on numbers. It dilutes the art and craft of meaningful composition to bean counting. It also confuses students. When I teach them how to format and paragraph dialogue many simply do not believe a line such as He shook his head. "No." deserves its own paragraph.
As teachers we'd find it much easier to teach some of the common core writing standards if we gave up some of our little white lies. Allowing students to fully explore a wide variety of sentence structures (including . . . gasp . .. starting sentences with conjunctions) will go a long way in meeting the following standards:
Use words, phrases, and clauses to create cohesion and clarify the relationships
among claim(s), reasons, and evidence.
Common Core Writing Standard 7.1.c
Use appropriate transitions to create cohesion and clarify the relationships
among ideas and concepts.
Common Core Writing standard 7.2.c
Think about it. Cohesion. Clarifying relationships. Subordinating and coordinating conjunctions are perfect for these tasks. And they can do their jobs just fine and dandy at the beginning of sentences.
Time for true confessions. What teacher lies have you told?