There is no proof. Your physics 1 class is correct, it is just defined.
What typically happens is that people play around with some equations and they find out that they have to keep on writing equations with terms of f.d. After writing f.d the tenth time they get tired of it, so they just make the substitution w=f.d, call it work, and presto work is defined. No proof is needed, it is just a definition.
What would be proven would be something like the relationship between work and energy.