It is impossible to score 15,000 points in a single race in Mario Kart Wii. The absolute maximum for one race is 12,000 points, achievable only with a perfect run.
Total: 1,000 + 1,000 + 10,000 = 12,000 points.
While a single race caps at 12,000, you can accumulate well over 15,000 points across the four races of a Grand Prix cup:
Even without a mythical 15k single race, consistent high-scoring races yield impressive Grand Prix totals.