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