Editing requires a thorough understanding of the English language. Writing and editing are closely interrelated. An editor has in all likelihood been a good writer with impeccable writing skills and a sublime sense of vocabulary. All these subliminal qualities including his innate proficiency at the use of the language has translated into him becoming a successful and a good editor.
Editorial skills are not taught, but developed through a rigorous involvement in the writing field and a first-hand experience at it. The use of grammar taught at schools does help you give a little insight into the working of the English language. Nonetheless, it is just an attempt or a course so that the pupil can learn to inculcate a sense of a grammatical feel and touch. This can help him in developing good language skills with a definitive mindset, which in the long run would be beneficial for him to pursue his desired writing-related ambition.
What training in this field would not teach you is to cope up with the burgeoning demand on creativity that this field demands. The ability to come up with a variety of a choice of words for the desired article to look pleasing enough to read is found in a good writer. The discerning capacity of the editor in selecting the right words that are suitable for the use of them in the context of the article and discarding the unnecessary words comes with the experience wrought in the field of writing from a good number of years.
A writer who makes steady progress in his understanding of the English language can vouch for the prospect of becoming an editor, when approached with a proposition for the same. This may seem as an ambivalent proposition to some and the writer may sulk in the vastness of the ocean of aspirants armed to the teeth with their impeccable language skills, but this option is worth the try.