I took AP human too. Yes, it's true that race does not have a clear scientific basis. For example: particularly in the US, the definition of what is "white" or "not white" has really changed over time.... when large numbers of Irish immigrants moved to the US, they were considered "dark." Certainly when Italians and eastern European Jews moved to the US in large numbers, they were not considered white. How would you describe the race of a Turkish person?
Also, in the US, what is black? Going back hundreds of years, there was no such thing as being "mixed." if you were 1/8 black and 7/8 white, you were legally considered black, and today that persists culturally, because nearly all Black Americans are mixed European and African heritage.
So basically the idea of race was a modern concept that there's a fixed and limited number of distinct human races, almost like different species. This debunked "scientific" theory was used by US and Europe to justify dehumanizing slavery of Black people and the forced removal of Native Americans from their home lands. In reality, there's a complete spectrum of differences among ethnicities around the world, and the differences are basically superficial, such as skin color.... ie, east Asian people are more likely to be lactose intolerant.
