For the composition component of our high school curriculum, we assume that the student has some experience writing essays. The three most important activities to help a student write well are reading good writing, writing as frequently as possible, and having his or her writing critiqued by an experienced writer or teacher.
If you feel that your student needs more guidance, you can find many aids to help you in teaching writing. The Online Writing Lab from Purdue University is an excellent resource. We have found The Elements of Style by William Strunk Jr. and E. B. White to be concise and helpful.
Other resources that have been recommended to us include The Elegant Essay by Lesha Myers, Format Writing by Frode Jensen, Writing Skills by Diana Hanbury King, and Teaching the Essay and Teaching the Research Paper by Robin Finley. The Institute for Excellence in Writing offers comprehensive instructional courses.