"Without undervaluing any other human agency, it may be safely affirmed that the Common School, improved and energized, as it can easily be, may become the most effective and benignant of all the forces of civilization."(Horace Mann)
I know what Mann is saying, but I can't put in my own words( so I am a little confused).
Please write what YOU THINK it says. Then someone here will be able to help you.
If schools are improved and motivated it will become society’s strongest quality
I'd choose a different synonym for "energize" but overall, this looks good.
http://thesaurus.reference.com/browse/energize
Horace Mann, an American educational reformer, is stating that while acknowledging the significance of other factors that contribute to societal progress, the Common School (referring to public education) has the potential to be the most impactful and positive force for civilization when improved and revitalized.
To put it in your own words, you can say that Mann believes that, even though there are other important factors involved in shaping a society, he emphasizes the immense power and potential of public education to be a force of positive change and progress. He believes that by enhancing and strengthening the Common School system, it can have a profound impact on the advancement and betterment of civilization.