Count days between two dates, conditional based on another table

Try

Num school days = SUMX( Student,
    var StartDate = Student[Current Entry Date]
    var EndDate = MIN( TODAY(), Student[Current Exit Date] )
    return CALCULATE(
        COUNTROWS( 'Date' ),
        'Date'[Date] >= StartDate && 'Date'[Date] <= EndDate,
        'Date'[Type] = "School day"
    )
)

Hi , ambere 

According to your description , you want to calculate school days enrolled by using the start and end date in table 1 (end date should be either end date or today, whichever is sooner.) 

Here are the steps you can follow:

(1)My test data is the same you provided.

(2)We can create a calculated column :

Days = 
var _exitdate=IF([Current Exit Date]>TODAY(),TODAY(),[Current Exit Date])
return
COUNTROWS(FILTER(Sheet1,'Sheet1'[Date]>=[Current Entry Date]&&'Sheet1'[Date]<=_exitdate&&'Sheet1'[Type]="School Day"))

(3)Then we can meet your need , the result is as follows:

If this method does not meet your needs, you can provide us with your special sample data and the desired output sample data in the form of tables, so that we can better help you solve the problem.

Best Regards,

Aniya Zhang

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