If you're in the US (I don't know about other countries) you can take Advanced Placement (AP) tests while in High School and then have those count for credit at many colleges. Most colleges will publish a list of which AP tests, with which scores, are accepted and the number of units/credit hours they are worth and the degree requirements they satisfy.
These tests are, in general, not very hard for above-average high school students. If you have taken a regular (non-AP) course, it should be possible to pick up the AP test study book, review for a couple weeks, and then pass the test.
As a personal anecdote, when I was in high school, I took biology my freshman year. My junior year, I picked up the AP study book and put in 2 weeks of work on it, then took the AP test, scoring a 5 (perfect score). This was worth 8 units (2 university courses, out of 180 units required to graduate) and satisfied some other requirements.
I was so happy with this discovery, that I did the same thing for many more classes the next year and entered college with 56 units (out of 180) complete, allowing me to graduate in 3 years and save almost $30,000 dollars.
Taking AP's on your own is totally doable. My school didn't offer any and I took six. In my experience, as nydwracu said, AP Psych is easy. So is Environmental Science and English Lit (if you do well on the english sections of the SAT). World History is interesting, and easy if you like memorization. I've heard Human Geography is easy too. The AP exams of languages that are commonly spoken as first languages in the US (Spanish, Chinese) tend to be harder than the ones that aren't (Latin, German) because native speakers drive the average up (it's not ...
It's that time of year when high school seniors are thinking about colleges, and by extension, everyone who knows any high school seniors is thinking about colleges as well. So let's let Less Wrong join in!
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