However, this is not close to the expected result. The data for the various grades comes from a single table instead of different tables, as listed in the original message.
The dashboard must allow the user to choose a start and end grade, which ought to filter the results and present the same student cohort data as an aggregate.
The resultant graph/table must show the aggregate value across only the students which exist in all the grades from the chosen grade.
Above you can see the plot of aggregates of scores for a subject across the grades. The score aggregate is for students who are the same across all grades over the years. It excludes students who joined in or left in the middle for the selected grade/period.