Ironfoot by Dave Duncan
Ironfoot is the first book in the Enchanter General series by Dave Duncan. It takes place in 12th century Norman-ruled England, an especially bad time to be a a poor, crippled, Saxon youth. But such is the lot of our hero, Durwin, nicknamed Ironfoot for the metal boot he wears to support an injured leg. By chance, his cleverness and competency with horses earns him a place as a student at a university which teaches the art of magic along with the sciences. When catastrophe befalls a professor’s noble family, Durwin is called upon to accompany him home.