Complex sentence: One of the abbey's apostolates is a school (Gymnasium) for the seventh to twelfth grades, which has existed in its present form since the 1840s or 50s, and is the continuation of a tradition of education that goes dates to the thirteenth century.
Split into simpler sentences: One of the abbey's apostolates is a school (gymnasium) for the seventh to twelfth grades which has existed in its present form since the 1840s or 50s. It is the continuation of a tradition of education that dates to the thirteenth century.