I'll go with Antares686 suggestion,
thus version II
Select Calc.StudentNr,Calc.ColSum/Calc.FieldCnt From (
Select StudentNr,
( ISNULL((CASE When ST1Q1 = 'N' Then Null ELSE ST1Q1 End),0) +
ISNULL((CASE When ST1Q2 = 'N' Then Null ELSE ST1Q2 End),0) +
ISNULL((CASE When ST1Q3 = 'N' Then Null ELSE ST1Q3 End),0) +
ISNULL((CASE When ST1Q4 = 'N' Then Null ELSE ST1Q4 End),0)),
( (CASE When IsNull(ST1Q1,'N') = 'N' Then 0 ELSE 1 End) +
(CASE When IsNull(ST1Q2,'N') = 'N' Then 0 ELSE 1 End) +
(CASE When IsNull(ST1Q3,'N') = 'N' Then 0 ELSE 1 End) +
(CASE When IsNull(ST1Q4,'N') = 'N' Then 0 ELSE 1 End))
From tblRCStudentGrades) as Calc(StudentNr,ColSum,FieldCnt)
Where Calc.FieldCnt>0