How can I make a time card field dependent to another time card field?

Summary:

Hi, we have tried to utilize value sets for this, but are not coming up with the expected results. Here is the scenario:

When a supply teacher selects "school" on the time card they are substituting in - the next field which is the "name of the person replaced" should only have values from the "school" that's been selected. I've attached a screenshot for reference.

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