Tables :: Automatically Fill Field Based On Keywords In Another Field

I'm trying to complete a database.

It is to manage details of pupils with additional support needs, and plan for the extra arrangements the school will provide for assessments.

It has 2 Tables

tbl-PupilDetails
-ScottishCandidateNumber primary key (Unique number which identifies pupils to the exams board)
-Forename
-Surname
-DOB
-YearGroup
-Class
-NatureOfNeed (memo)
-EvidenceOfNeed (memo)

tbl-SubjectLevelArrangement
-ID Primary key, Autonumber
-Pupil foreign key to tblPupilDetails
-Subject
-Faculty
-Level
-Arrangement

I currently use forms for adding new pupils, and updating pupil subjects/arrangements/levels.

I also have forms to search for specific pupils, and to create lists for faculty heads showing which pupils are taking subjects within their faculty and the arrangements we expect to provide.

I use the forms to run queries, which can then output to reports for printing.

Where I am currently having an issue is the faculty field in the tbl-SubjectLevelArrangements. (If I didn't have to report to faculty heads I would just leave it out, but management will insist.)

Currently I have a form with dropdowns for adding subject, faculty, level and arrangement manually. This is acceptable for the subject, level and arrangement because they are completely interchangeable and dynamic throughout the academic year as pupils may drop down a level, or change the type of arrangement they require.

However as faculties are inextricably linked to subjects, I want to remove the possibility of human error. i.e. when a user (me) chooses either geography, history, or RE, then the faculty will always be Humanities, likewise if the user chooses French, German, or Spanish, then the faculty can only be Modern Languages etc.

I'm convinced there must be a very simple way to ensure that the faculty field prefills based on the limited keywords available in the subject field, but I just cannot figure it out.