Your mediocre teach at your little public school was right. Time changed things. One should always be wary of assigning causality to historical events, particularly big, nebulous concepts like "the middle ages" or "the renaissance," just as one should be wary of criticizing one's teachers.

Neither the middle ages nor the renaissance were time periods or events recognized by contemporaries, who would have referred to themselves as "modern." Indeed, the current idea of the "medieval period" was created over the course of the sixteenth and seventeenth centuries as a juridical means to separate contemporary society from the past. "The renaissance" got tacked on as a way to transition from a backwards past to a glorious present.

I could list off several dozen things that helped refocus the milieu of the upper class around humanism and the "rediscovery" of antiquity, from the Byzantine decline following the Fourth Crusade to the creation of paleography as a means of making legal arguments. All of these may have contributed to change, but none "pushed" it, because events do not have agency. Millions of individual responses to localized stimuli "push" change over time. Or, as your teacher put it, time changes things.

For the seventeenth century creation of the medieval, see Kathleen Davis' Periodization and Sovereignty.