The first time you refuse to be talked down to at work, even politely, you learn something solid about yourself.

Being spoken to as though you are less than a person is common in first jobs, and the usual response is to absorb it. The discovery comes the first time you decline without making a scene, in an even voice, and the ground does not open. Standing up and being polite turn out not to be opposites.

Have one sentence ready in advance, since inventing it on the spot is harder. Something like asking to be spoken to normally, or saying you will do the task but not be shouted at. Say it level, carry on working, and notice how much steadier you feel.