You are Correct. it is the FIXED LOD which is causing it
Is there another way to sum the transaction amounts by school?
You blended the data sources - is there any way that you could have used a Join or a Union to connect the data sources -
When you blend the data sources you will not be able to mix data from the 2 sources in an LOD expression - not an issue if you could join the data
If you could attach your workbook with the data (even dummy data) I will look at it
sorry but without seeing the data I would just be guessing at a solution - not very productive