creating a rolling average

Hi, i have a table displaying the weekly average attendance of a student:

It is calculated using a 'rolling' average, so:

week 1 actual (05/09/2022) = 100%

week 1 rolling (05/09/2022) = 100%

week 2 actual (12/09/2022) = 90%

week 2 rolling (12/09/2022) = 95% (100+90 = 190, /2  = 95%)

week 3 actual (19/09/2022) = 100%

week 3 rolling (19/09/2022) = 96.67% (100+90+100= 290, /3  = 96.67%)

The DAX for this measure is:

Weekly % Attendance (Rolling) =

AVERAGEX(

    FILTER( ALLSELECTED( 'powerbi_attendance_weekly_average' ),

        'powerbi_attendance_weekly_average'[Mark Date (Weeks DD/MM/YY)] <= MAX (powerbi_attendance_weekly_average[Mark Date (Weeks DD/MM/YY)])),            'powerbi_attendance_weekly_average'[Weekly % Attendance])

This seems to work fine in a PBI table when the table is filtered to a single student, as above, but when I try to display multiple students, this is the result:

I suspect the value is a combined value for all students. These are fictitious names, I should add.

Hope somebody can give me some pointers with this, many thanks