AP Classes: What They Are and Why You Should Care

"Since many high schools offer AP classes to their seniors, or to their juniors and seniors, you should gain some understanding about the significance of those classes during your sophomore year. That way, if you're interested in taking an AP class, you'll have plenty of time to talk to your counselor before you select your 11th grade classes."

"AP classes are taught at a higher level than normal high school classes. When taught well, they are as challenging as many freshman-level college classes. For example, AP U.S. History is designed to be as challenging as a freshman U.S. history class at the typical college."

From the Princeton Review