Take another look at the original sentence:

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.

Go to the next sentence.

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