Final answer:
The 10 percentage point increase in test scores at a high school with a longer school day may be due to self-selection of motivated students, not only the extended hours.
Step-by-step explanation:
The improvement in student performance on statewide tests after extending the school day from 6.5 to 7.5 hours does not conclusively prove that a longer school day improves student achievement. This is because the improvement could also be attributed to self-selection, where only motivated students who were willing to sign a contract attended this school. These students may have performed well regardless of the length of the school day. The simultaneous changes make it difficult to isolate the specific factor that led to the improved test scores.