You’d better straighten up!
What does “straighten up” mean here? Does it mean “sit straight”,”stand straight” or “be better” according to the context below?
The Word That Made a Difference
How just one word of praise helped a shy student break through a mental block.
Sixth grade hadn’t been a banner year for Eric. Never very confident in school, he had a particular dread of mathematics. “A mental block,” one of the school’s counselors had told him. Then, as if a mental math block wasn’t enough for an eleven-year-old kid to deal with, he came down with measles in the fall and had to stay out of school for two weeks. By the time he got back, his classmates were multiplying fractions. Eric was still trying to figure out what you got when you put a half pie with three-quarters of a pie...besides a lot of pie. Eric’s teacher, Mrs. Gunther—loud, overweight, and terrifying—was unsympathetic. For the rest of the year she hounded him with ceaseless makeup assignments. When his mental block prevented his progress in fractions, she would thunder at him in front of the class, “I don’t care for your excuses! You’d better straighten up!”