Declare and and define Property in VB - vb.net

Question

I have two Class and I want to save my data into arrays form text box like this:
Students.Name(txtID.Text-1).MathMark = txtMark.Text
but I get error: Object reference not set to an instance of an object
my code is:
Dim StudentsNumber as Integer = txtstdnum.Text
Dim Students as New StudentsInf(StudentsNumber)
Students.Name(txtID.Text-1).MathMark = txtMark.Text
Public Class StudentsInf
Private mName() As String
Sub New(ByVal StudentNumbers As Integer)
ReDim mName(StudentNumbers-1)
End Sub
Public Property Name(ByVal Index As Integer) As LessonsMark
Get
Return mName(Index)
End Get
Set(ByVal Value As LessonsMark)
mName(Index) = Value
End Set
End Property
End Class
Public Class LessonsMark
Private mMathMark() As Object
Public Property MathMark() As Object
Get
Return mMathMark
End Get
Set(ByVal Value As Object)
mMathMark = Value
End Set
End Property
End Class

Answer 1

This:
Private mName() As String
needs to be:
Private mName() As LessonsMark
then you have to create the objects in your constructor, something like:
Sub New(ByVal StudentNumbers As Integer)
ReDim mName(StudentNumbers - 1)
For i As Integer = 0 To StudentNumbers - 1
mName(i) = New LessonsMark()
Next
End Sub
then it looks like your LessonsMark class is declaring an array of objects when it looks like it should be just a string property:
Public Class LessonsMark
Private mMathMark As String
Public Property MathMark As String
Get
Return mMathMark
End Get
Set(ByVal Value As String)
mMathMark = Value
End Set
End Property
End Class