A change in the laws is evidence that changes in attitudes, beliefs, and expectations have taken place on some level. People must obey the law even if they do not agree with it (for instance, same sex marriages). Future generations will be able to look back on discriminatory policies and laws and see how they have been discontinued or even reversed.
Meaningfully lower rates of poverty in our country would be the first type of progress to make be think we're getting somewhere.
If I were a high school student, I think I would probably focus on education. Higher graduation rates from high school and college, lower costs in higher education, and a wider set of the ideas of what is possible, what is out there, and what is do-able for college graduates would all be hopeful signs.
Two of the most important kinds of change it would be good to see in the next decade or so is a steady diminishment of the national debt and realistic solutions to the problem of unfunded liabilities. The U. S. spends (or commits to spending) much more money than it takes in. I am not sure, frankly, how or even if these problems can be solved.
This depends on what area we're talking about. Maybe they would need to see reduced inequality. Maybe they would need to see that gay marriage was legal. Looking at it from a more conservative point of view, maybe they would need to see abortions go way down. Or maybe they would need to see lower rates of divorce and of childbirth out of wedlock.