An Education Plan includes a free Workspace with 5GB of storage and a number of editor seats (depending on your education status) — that means free access to the Mac app.
Students and Teachers are entitled to a Workspace with 1 Editor seat, and Institutions are entitled to a Workspace with multiple seats — depending on requirements.
So, if you’re a student or teacher, you can use Sketch for free for as long as it’s part of your studies or the course you’re teaching. Similarly, if you represent an educational institution, you can freely offer Sketch to your students as part of your syllabus.