Madget,
According to the investigative work of vicente y exxcentrica, it seems like "Bachillerato" is the last two years of secondary education.
Equivalent to the last two years of High school in the US, where they are called the Junior, and the Senior years. Ages 16 - 18
And I wonder if it is also equivalent to the last two years of secondary education in England at ages 16 - 18. It is called "A Levels", and without it you can not go to university.
It is interesting to notice that although these are the equivalents according to ages, they differ vastly in content.
Facilisimo, no?
Kelly