Take a look at the original sentence again:
For two weeks, Todd refused to go to his accounting class because Professor
Gastineau would not allow students to wear hats in
class, under no circumstance would Todd
allow anyone to see the butcher job his stylist had done to his hair.
You chose to fix it the correct way:
For two weeks, Todd refused to go to his accounting class because Professor
Gastineau would not allow students to wear hats in
class, and under no circumstance would Todd
allow anyone to see the butcher job his stylist had done to his hair.
For two weeks, Todd refused ... begins a complete sentence. Under no circumstance would Todd allow ... starts the second complete sentence. A comma and the coordinating conjunction and will fix the problem nicely.