Final answer:
A definitive maximum of classroom instruction per day cannot be established without considering local policies; also, longer school hours alone cannot be credited definitively for enhanced student performance without a comprehensive study.
Step-by-step explanation:
The maximum amount of classroom instruction that can be given in one day varies and is subject to individual school policies, state regulations, and country laws. However, your question seems to be probing the relationship between the length of the school day and student achievement, rather than a straightforward maximum. In the example you've mentioned, where a high school increased the school day from 6.5 to 7.5 hours and saw an increase in statewide test scores, one cannot definitively claim that the longer school days improve student achievement based solely on that correlation. There could be a variety of factors contributing to this increase in performance, such as the contract system encouraging commitment or the quality of instruction. Additionally, a longer school day might contribute to better academic performance, but a comprehensive study would be required to prove this conclusively.