It is probably true that Communist Cuba destroyed its education system less than it destroyed other things. But if you look on an absolute scale, the average education of the Cuban population has been lagging.
Some of the lag is due to the fact that Communism either killed or drove away comparatively educated people. But if that is masking Castro's success, that's no success!
Update: The Barro-Lee data say that, as of the year 2000, the majority of adults in Cuba had NOT graduated high school. At the same time, the majority of adults in Puerto Rico HAD graduated high school (Table 5-3 of this book).