Amanda,
As Michal said, you will want to create a definition query to display only the unified school districts:
1. Right click the layer in the table of contents > Properties > Definition Query > Query Builder...
2. Create an SQL statement that says something like "TYPE" = 'unified'
3. Click OK
Now only your unified school districts should display. To symbolize these remaining records based on your tax rate:
1. Right click the layer in the table of contents > Properties > Symbology
2. Click Quantities > Graduated Colors
3. Under Fields, for Value, select your tax rate field
4. You can change around the classification method by clicking Classify.
Luke