You first need to create a proper array that has some elements, for example:
const int STUDENT_COUNT = 20;
int studentArray[STUDENT_COUNT];
Then you need to populate it with the scores, you could do it manually
int studentArray[STUDENT_COUNT] = { 21, 13, 20, 20, 14, 7, ... };
or by using some function that generates random numbers
for (int i = 0; i < STUDENT_COUNT; ++i)
{
int score = GetRandomScoreValue(); studentArray[i] = score;
}
etc.
Then all you need to do is iterate the array looking for the highest and lowest scores. Then do the same again counting the number of students who have reached each score. And finally print your results.