Answer:
Step-by-step explanation:
The whole point of Walden is to say what truly reflects who you are, not what you think others want to hear, including Thoreau.
Your teacher has left you no room to disagree, if indeed you do disagree. The point is that you might. And Thoreau would be the first to applaud you for doing so.
So this quotation states in unequivocal terms that you must state what you mean and not hide behind some washed out statement that is meant to hide what you really think.
The only exception I can think of would be some opinion that if you don't lie, your life is in great danger. Crossing into an unfriendly border might mean death if you don't lie. For example during WWII, crossing into the Swiss border from Germany when asked if I was Jewish, I would deny my heritage in no uncertain if I was Jewish.
So there are exceptions, but by and large, principle is more important than being uncomfortable.