Create a Calculated Column from Different Tables

Thank you for the information. I tried this but now I'm getting a different error - A single value for the column can't be determined. I may have a bigger issue. Here is what I am trying to do:

I have one table listing school districts and building codes (multiple buildings per district) and another table with the pupils per district per year:

Table 1                                                  Table 2

School    Building Code                        School   Year   Pupils

AB          1000                                        AB          2018  200

AB          1001                                        AB          2017  300

CD          1002                                        CD         2018  400

CD          1003                                        CD         2017  500

etc                                                          etc

Finally I have a large table with all the raw financial information for all the districts, every transaction with a column for the year and the building code. I can do great charts of the financial information by school but I want to include a calculation of expenses per pupil. I want to be able to drill down to look at things like instructional expenses per pupil or transportation expenses per pupil in any given year. It seems like the only way to do this is to get a column in the financial information table which divides each transaction by the number of pupils per school, and the only way to get that is through the building code and year. Is there a formula which would look at the year and building code column in each row, then pull in the appropriate number of pupils?