Here, try this;
Assume that for each of the years you provided data, there are the following graduations and admits to start the following Fall.
Grads: 2, 1, 3, 2, 2, 0, 3, 0, 1, 1, 4, 3, 2, 6, 9
Admits: 3, 1, 3, 0, 1, 3, 0, 6, 4, 13, 1, 4, 6, 3, 0
I assumed there were your 12 in 98-99, so 2 graduations and 3 admits would give you 13 on campus for the next year. For the next year there was 1 and 1 and so on…