The authors specifically said that the elementary schools sampled at Time 1 fed into the high school at Time 2, but they didn't say that the data was paired ie:

The authors specifically said that the elementary schools sampled at Time 1 fed into the high school at Time 2, but they didn't say that the data was paired ie: the set of students sampled at Time 1 = the set of students sampled at Time 2. The response rate is so high that there is likely considerable overlap tho.