Make code faster/better best practices

A couple of changes to ignore case in name comparisons, and to avoid an error if the number of adds or drops is zero:

Sub AddRem(rCur As Range, rOld As Range, rAdd As Range, rDrp As Range)
    Dim nCur    As Long     ' current students
    Dim nOld    As Long     ' prior students

    Dim asAdd() As String   ' array of new
    Dim asDrp() As String   ' array of drops

    Dim iCur    As Long     ' index to current students
    Dim iOld    As Long     ' index to prior students

    Dim nAdd    As Long     ' number of new students
    Dim nDrp    As Long     ' number of drops

    nCur = rCur.Count
    nOld = rOld.Count

    ReDim asAdd(1 To nCur)    ' max  adds is everyone here this year
    ReDim asDrp(1 To nOld)    ' max drops is everyone here last year
    
    ' clear output area
    rAdd.Resize(nCur).ClearContents
    rDrp.Resize(nOld).ClearContents
    
    ' sort both lists
    rCur.Sort Key1:=rCur(1), Order1:=xlAscending, MatchCase:=False, Header:=xlNo
    rOld.Sort Key1:=rOld(1), Order1:=xlAscending, MatchCase:=False, Header:=xlNo

    iCur = 1
    iOld = 1
    Do
        Select Case StrComp(rCur(iCur).Text, rOld(iOld).Text, vbTextCompare)
            Case -1    ' cur < old; current is new
                'rCur(iCur).Select   ' debug
                nAdd = nAdd + 1
                asAdd(nAdd) = rCur(iCur).Text
                iCur = iCur + 1
            
            Case 0  ' cur = old; continuing student
                iCur = iCur + 1
                iOld = iOld + 1
            
            Case 1  ' old < cur; old is a drop
                'rOld(iOld).Select   ' debug
                nDrp = nDrp + 1
                asDrp(nDrp) = rOld(iOld).Text
                iOld = iOld + 1
        End Select
        If iCur > nCur Or iOld > nOld Then Exit Do
    Loop

    ' add any remaining current students to add list
    For iCur = iCur To nCur
        nAdd = nAdd + 1
        asAdd(nAdd) = rCur(iCur).Text
    Next iCur

    ' add any remaining old students to drop list
    For iOld = iOld To nOld
        nDrp = nDrp + 1
        asDrp(nDrp) = rOld(iOld).Text
    Next iOld

    ' output each list if not empty
    If nAdd > 0 Then
        ReDim Preserve asAdd(1 To nAdd)
        rAdd.Resize(nAdd).Value = WorksheetFunction.Transpose(asAdd)
    End If
    
    If nDrp > 0 Then
        ReDim Preserve asDrp(1 To nDrp)
        rDrp.Resize(nDrp).Value = WorksheetFunction.Transpose(asDrp)
    End If
End Sub