One of the great strengths of homeschooling is that it gives parents the ability to tailor their children’s education to their individual needs. Although Exploring the World through Story was originally designed as a K-8 program, placement is flexible; the texts in the lower levels appeal to wide range of ages, and the upper levels of the program provide plenty of challenge for high schoolers. The EWS placement guide lists the reading levels, writing skills, and prerequisites for beginning each level, and I encourage parents to use those skills, rather than grade levels, when deciding what level will work best for their child.
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.