One of the great strengths of homeschooling is that it gives parents the ability to tailor their child’s education to their individual needs. Although Exploring the World through Story was designed as a K-8 program, placement is flexible. EWS provides carefully scaffolded writing instruction and stories that appeal to a wide range of ages, so the suggested grade levels are just that—suggestions. The placement guide lists the writing skills taught at each level, and I encourage parents to use those skills, rather than grade levels, when deciding what level will work best for their child.
So, yes! You can use the middle school levels of EWS for your high schoolers who need an introduction to essay writing. Most students in this situation will benefit from the EWS Writing Skills for Older Beginners program, which takes students ages 11+ through the foundational writing skills taught in EWS Levels A-F. After that, they can proceed to Level G and on through the rest of the middle school levels.
If you want to assign high school credit for EWS, consider the amount of time your student is spending on all of their ELA work. To earn one Carnegie unit, a high school student must have 120 hours of classroom time in an academic year. For a homeschooled student, this averages out to 45 minutes of ELA a day, five days a week, for 32 weeks. If you follow my recommendation to add a formal grammar curriculum to EWS, your student should easily meet or exceed these requirements.