What is the thesis statement of the speech "The Necessity of Education in a Republic Government" (fall 1839) by Horace Mann?

Mann's thesis is really implicit—he even remarks at the beginning of the speech that he will not repeat it—but a summary of his argument can be found in the speech. Essentially, he argues that education is more important, essential even, to republican government than it is to a despotic government. This is because republican governments, being characterized by freedom and individual liberty, allow people some room to indulge their passions and prejudices. In short, Mann says, people are allowed to give vent to the negative side of human nature as well as its positive aspects. Therefore, Mann argues, republican governments have the imperative to counsel, train, and teach people to do good rather than evil:

If republican institutions do wake up unexampled energies in the whole mass of a people, and give them implements of unexampled power wherewith to work out their will; then these same institutions ought also to confer upon that people unexampled wisdom and rectitude.

In other words, if people are going to be free to do bad things, as they are in a republican form of government, they need to be trained to do good things. If a republican government fails to do this, he says, it will perish:

If we maintain institutions, which bring us within the action of new and unheard-of powers, without taking any corresponding measures for the government of those powers, we shall perish by the very instruments prepared for our happiness.

Unlike despotic governments, which crush the free will of people, free governments allow people to act in ways which may not be good—indeed may be destructive to—the common good. Education, in Mann's view, consists of cultivating people to harness their energies and their impulses for good. This is crucial in a free society.
https://voicesofdemocracy.umd.edu/horace-mann-the-necessity-of-education-in-a-republican-government-speech-text/