How to avoid class instances from interfering with each other

I have an application in which I've created a class named 'Student' used to hold students details. I have declared it private ostudent in two different forms, say form1 and form2, both of which are called from a MDI form. When I open both forms and select different students in each form, I find that the instance selected first changes to the details of the student selected last. Here is a code excerpt from the two forms

Public Class FrmDiscontinue
    Private oStudent As Student

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        StudentID = ""
        Me.oStudent = New Student
        nTransferYear = NumericUpDown1.Value
        nTransferTerm = ComboBox1.SelectedIndex
        Dim fs As New FrmSearch With {.SearchCriterion = 26}
        fs.ShowDialog(Me)
        If Trim(StudentID).Length > 0 Then
            Me.oStudent = oMain.GetStudent(StudentID)
        Else
            MsgBox("No student selected", MsgBoxStyle.Information, Text)
            Return
        End If
        TextBox1.Text = Trim(Me.oStudent.ToString)
    End Sub
End Class

Code from the second form is below

Public Class FrmAdminStudentDetails
    Private oMain As New MainClass
    Private cDetailSet As New DataSet
    Private oStudent As Student

   Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Me.oStudent = New Student
        StudentID = ""
        Dim fs As New FrmSearch With {.SearchCriterion = 0}
        fs.ShowDialog(Me)
        If Trim(StudentID).Length > 0 Then
            Me.oStudent = oMain.GetStudent(StudentID)
            TextBox1.Text = Trim(oStudent.ToString)
        End If
    End Sub
End Class

What happens is that when both forms are open, when I select a student on one form, then go to the second one and select a different student, oStudent becomes the same in both forms

Student class code excerpt is below

Public Class Student
    Private stdid

    Public Sub New()
        stdid = "" 
    End Sub

    Public Sub New(ByVal cstdid As String)
        stdid = cstdid 
    End Sub

    Public Property StudentID() As String
        Get
            Return stdid
        End Get
        Set(ByVal value As String)
            stdid = value
        End Set
    End Property
End Class