Writing opens the door to publicity and persuasion, research dissemination and criticism, many kinds of leadership in education and elsewhere. There are many excellent materials on writing for adults. From what I can see, I think the main step in writing better is to write more. It is common for educated people today to bemoan the state of language, particularly writing skills, in today's young people. Similarly, when journalists want to make teachers look bad, they like to obtain a piece of writing by a teacher which contains non-standard spelling or grammar and publicize it.
Many people tend to equate writing skills with good spelling and punctuation.
Experienced teachers of writing have long concentrated instead on stimulating and encouraging the student to get ideas in some form or other on paper. Correcting spelling and punctuation can be done later. Of course, correction and editing do need to be done if the writing is going to be accepted and not be an embarrassment. My experience faults the development of such voice in schooling at all levels far more than spelling or punctuation skills. Email promotes speedy composition and encourages the sending of messages which have not be slowly composed nor painstakingly revised.