I disagree with teachers publicly sharing scores (in most cases), but every kid in our district knows that your score flashes up on the screen at the end of your MAP test. Can't really hide the score from the kid. DD10 tells me her score after she tests - you don't need to wait until the scores come home.
FYI, this showing of the score at the end of testing is a feature that can be turned off by the administration. My son told me in 5th grade that the teacher turned off the scores because the kids were comparing scores too much.
Note that the growth in MAP scores is designed to go up more dramatically in the early grades, and then it decreases as the students get older. That is another possible reason you won't see quite the same giant leaps in growth after the first couple years.
These are the winter RIT math scores from kindy through G11, 99th %ile. See how the difference between higher grades is only a couple points, whereas lower grades jump ~15? (Ignoring here the fact that you can get higher scores than 284)
182 - K
205
217
230
243
254
262
272
277
281
283
284 - 11th
NWEA 2011 RIT Scale Norms - see App B