I heard sum1 got 100% on the test, though I don't know how that is possible.
|SOS||Someone over shoulder|
|SIA||Someone is around|
|SOSG||Someone over shoulder gone|
|FAWC||For anyone who cares|
Sum1 is a creative, unusual way to abbreviate "someone." Sum1 who sends you this abbreviation is likely a master chatter, who wants to show off their text-fu.
Now that you know about sum1, you can use it yourself in text and chat messages. That way, all your friends will think you're super kewl.