Why do the school grades stop at 12 and not a grade 13?

The school grades stop at 12 because that's the point at which most students complete secondary education and are eligible for post-secondary options like college or university. Grade 12 (or its equivalent) marks the traditional completion of K-12 schooling. There's no further need for a numbered grade within the standard secondary school system after that point. Post-secondary education is structured differently, with years of study rather than numbered grades.