Here we can see that this school is probably making pH measurements using pH paper. This explains the seemingly unusually high degree of jumps in pH with time. It is entirely possible that the actual pH of the water source being measured by this school in the midwestern United States is somewhere between pH 7 and pH 8. We would expect slight changes in water pH to push the readings back and forth between two values.
In the graph above, we see an example of a GLOBE school that is using a pH meter to conduct their measurements. Overall in this graph, the pH readings paint a picture of a water source whose pH is changing dramatically. The temperature data show a reasonably smooth temperature progression.