1. You're not gonna doing your homework but going out for shopping. 2. He/She didn't go to college after graduating from high school. 3. It means someone failed to pass an exam.

“上”alwasy used after the verb meas the result"结果", so 考上means he passed the exam, if he failed, we can say 没考上。 there are some other situations,we use “上” to mean the result, 赶火车（catch up the train）， 赶上，没赶上。追上，没追上（when you want to catch up somebady） hope this can help you to understand.

1 Don't do homework,let's go shopping 2 he (she) doesn't pass the college entrance exam when he(she) graduate from a high school 考上:pass the exam,then he(she) can get college passport,if he(she)doesn't pass the exam,he(she) can't study in university Sorry for my poor English,hope you can understand it

it means pass the exams to get into university :)