That's a somewhat odd way to structure an assignment: You're required to take a particular stand even if you're not sure you agree with it. I guess there can be value in trying to argue for opposing views to make sure that you have addressed the best arguments... but still, I worry a bit that your teacher might be trying to push a particular opinion rather than letting you figure it out on your own.
That said, there are plenty of ways in which the United States does not live up to its ideals of freedom, equality, and justice. I study global inequality, so I happen to know that by world standards we do moderately well. But by comparison to our own ideals or even to other First World countries like France, Germany, the United Kingdom, Sweden, and Norway, the United States still has a long way to go.
Some of the worst violations of freedom and justice in the United States are related to the so-called "War on Terror", the massive (over)reaction to the 9/11 terrorist attacks in 2001 that killed almost 3,000 Americans. In response to that watershed event, we in the United States expanded our military and intelligence services, militarized our police forces, and curbed a number of different civil liberties in the name of fighting terrorism. There are now many forms of searches and wiretaps that the US government performs without warrants, and in rare circumstances our government even kidnaps potential terrorists, moves them off our soil to Guantanamo Bay (a US-controlled region of Cuba) and tortures them---all without a warrant, let alone a trial. We now live in a world in which the NSA is very likely reading this very paragraph and running it through algorithms to determine if I am a potential terrorist. A man named Edward Snowden used to work for the NSA until he couldn't tolerate it anymore, and revealed these horrific surveillance systems to the world---only to be branded a traitor and forced to flee to Russia.
Of course, I'm White (mostly) and very definitely not Muslim, so the algorithm will probably give me a pass as someone who cares about civil liberties rather than a terrorist sympathizer. This brings me to the next major source of injustices in the United States: bigotry. While we have made substantial progress over time, from banning slavery in the 1860s and giving women the right to vote in 1920 all the way up to banning racial segregation in the 1960s and legalizing same-sex marriage in 2015, there still remains a large amount of racism, sexism, and homophobia in this country. African-Americans and Native Americans bear the worst racism, but others are still discriminated against in various ways. Because the 9/11 terrorists were motivated by an extremist Islamic ideology, most Americans are now at least suspicious of Muslims, and many are outright bigoted toward them. Many Muslims have faced ridicule, humiliation, or even violence as a result.
People from groups that are discriminated against systematically have worse outcomes in their lives, including less education, poorer health, and lower incomes. And this brings me to the final reason why the United States fails to live up to its ideals of equality and justice: Economic inequality. The United States actually has some of the most extreme income inequality in the world, meaning that some Americans are very poor while a few are fantastically rich. The top 1% of our population receives over 20% of the income. The top 0.01% receives almost 4% by themselves. We have over 500 billionaires---people so rich they could buy a Ferrari every day for the rest of their lives---but also over 50 million people in poverty.
To be clear, the United States does have many fine qualities, and our aspirations toward freedom, equality, and justice are not entirely in vain. We do have one of the highest freedom of speech ratings in the world, for example. Our elections are almost entirely fair and legitimate, and in the rare cases where someone tries to rig them the punishment is swift and severe. While bigotry still remains embedded in our culture, there have also been very large improvements in the quality of life of many oppressed groups in a remarkably short period of time. We are in fact the first White-majority country to elect a non-White head of state (President Barack Obama).
Even what we call "poverty" in the US is not as bad as poverty in many other countries, including Costa Rica where you are from. The US poverty line is about $11,000 per year while the poverty line in Costa Rica is only about $1,200 per year. I feel awkward admitting that what we call "poor" may seem fairly well-off by the standards you are familiar with. Our prices are higher here, especially in terms of housing, so money does not go as far... but even adjusting for that this remains true. Europe has an even higher poverty line, so what they call "poverty" honestly isn't; it's more like being less rich than average.
I hope that gives you enough to work from!