I think that Realschulabschluss comes a bit closer if you want to adapt the degree to comparable german schooling system. If you use it for translating a CV i would stick to High School Abschluss, since the person was in High school. But if you are translating a text which needs adaptation to the german school system than Realschulabschluss or "Mittlere Reife" would fit better in my opinion