How To Update An Existing LINQ Entity

How can I update an existing LINQ entity? This code gives me: "Cannot insert an entity that already exists."
Any better practises on updating/inserting with linq?

foreach (ListViewItem lvi in lvStudents.Items)
{
HiddenField hfStudentID = lvi.FindControl("hfID") as HiddenField;
CheckBox cbPresent = lvi.FindControl("mycb") as CheckBox;
int sInt = Int32.Parse(hfStudentID.Value);
dc = new DataClassesSODataContext();//I renewed the dc to make sure that wasn't the problem?
var currStudent = dc.students.Single(x => x.studentID == sInt);
grade g = dc.grades.Single(x => x.subjectID == subjID || x.studentID == sInt);
if (g == null)
{
g = new grade();
g.subject = dc.subjects.Single(x => x.subjectID == subjID);
g.student = currStudent;
g.present = cbPresent.Checked;
dc.grades.InsertOnSubmit(g);
} else
{
g.present = cbPresent.Checked;
dc.grades.Attach(g, true);//I tried both attaching and inserting
}
}
dc.SubmitChanges();