got it! :-) I simply divided sumproduct by the number of matched students:
=(SUMPRODUCT((OFFSET(OFFSET(grade!$A$3,0,0,student_no,1),0,MATCH($A$3,grade!$A$2:$DS$2,)-1)=$A$4)*(OFFSET(grade!$A$3,0,0,student_no,1)<>"")*(OFFSET(grade!$D$3,0,0,student_no,1)="f"),OFFSET(grade!$DX$3,0,0,student_no,1)))/
SUMPRODUCT((OFFSET(OFFSET(grade!$A$3,0,0,student_no,1),0,MATCH($A$3,grade!$A$2:$DS$2,)-1)=$A$4)*(OFFSET(grade!$D$3,0,0,student_no,1)="f"))