In China, schools usually don't talk about diversity, since uniformity and homogeneity are generally promoted in China, and there is only one race -- Asian. However, diversity is a crucial topic in the United States. There are so many races, languages, cultures, etc. And schools usually incorporate diversity into their curriculum or extracurricular activities.
There are some important topics related to diversity, including race, gender, sexual orientation, socio-economic status, ability, etc. The main idea is to promote equality and inclusion of different people. Unlike the situation in China where there is no racial discrimination (because there is only one race), socio-economic status does not matter too much in high school (since it has a minor effect on the Gaokao exam), and topics related to gender and sexual orientation are not brought up (as same-sex marriages are illegal and only two genders are officially recognized), there is a more diverse population of people in the United States, and some people were marginalized or disadvantaged in history because of their identities. Issues related to diversity are heavily influential in their lives.
Some relevant activities in high school include pride club (related to LGBTQ+ issues), MLK club (related to people of color), different culture clubs, diversity conferences (where people are educated about diversity and relevant discussions are held). If you are confused about something, the best thing to do is to ask teachers whose work is about diversity or search on the internet. You can also ask your American friends about these topics, but you need to be very polite and careful in crafting your questions and stating your opinions.
In China, biases on LGBTQ+ people and black people are popular, partly because of the Chinese culture which promotes homogeneity. It is not the right thing to do, but you can post your opinions on these groups on social media or talk about it in public. However, in the United States, you can be penalized for stating your bias and stereotypes.
Here are some great resources to learn about diversity.