Horace Mann is an event unique to 
USA.
It requires that USA is a democracy and can fire between 1838 and 1842 and has a mean time to happen of 5 months. That is reduced by 25% after 1839 and again by 5% after 1840. Unlike what would be logical, it does not require, that USA actually owns Massachusetts.
It gives 2 prestige, 1 plurality and 0.5 years worth of research points.
Ingame text
Politician and Lawyer Horace Mann began his reforms of the education system of Massachusetts. The system of schools became a model for the entire United States