in MS Access 2010 prevent Append query from creating duplicate entries

Append and update are two different operations.  Append adds rows to a table.  Update modifies existing rows.  To prevent "duplicates" from being added to a table, create a unique index on the combination of columns that defines uniqueness so following the directions to create the composite index was correct.  Please post a picture of the Index dialog so we can see what you chose.  Make sure the first line is highlighted so we can see the properties and make sure we can also see the table.

To handle the late comers, create a form that selects the attendance records for today and then scroll to the record you want to update and check the attended box.

I agree with Pat that you should have a unique index on the attendance data and the studentID.

But to go a step further, and only attempt to append records which do not already exist, thereby preventing an error from occuring.  I generally do this with a combo box (or possibly a textbox) to select the date.  Then, in the AfterUpdate event of that control, I insert all of the students into the Attendance table (assumes field ID, Att_Date, StudentID, IsPresent) based on a query which might look like the following:

INSERT INTO tblAttendance (Att_Date, StudentID, IsPresent)
SELECT S.Att_Date, S.StudentID, S.IsPresent
FROM (
SELECT Eval([Forms]![YourFormName]![cboDate]) as Att_Date, [StudentID], 0 as IsPresent
FROM tblStudents WHERE IsActive = -1
) as S
LEFT JOIN tblAttendance as A
ON S.Att_Date = A.Att_Date AND S.StudentID = A.StudentID
WHERE A.ID IS NULL

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This query starts out with a subquery that joins the date selected on your form to the list of active students, it then uses the LEFT JOIN to link to the current Attendance table and filter out those records which already exist, then it inserts the remaining records into tblAttendance.

if you set the default value of the IsPresent field in your Attendance table to False (0) then you can remove the references to the IsPresent field like:

INSERT INTO tblAttendance (Att_Date, StudentID)
SELECT S.Att_Date, S.StudentID
FROM (
SELECT Eval([Forms]![YourFormName]![cboDate]) as Att_Date, [StudentID]
FROM tblStudents WHERE IsActive = -1
) as S
LEFT JOIN tblAttendance as A
ON S.Att_Date = A.Att_Date AND S.StudentID = A.StudentID
WHERE A.ID IS NULL

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Then I would have a subform which displays all of the students with the IsPresent field as a checkbox based on a RecordSource of:

SELECT A.IsPresent, S.Student_LastName & ", " & S.Student_FirstName
FROM tblAttendance as A
INNER JOIN tblStudents as S ON A.StudentID = S.StudentID
WHERE A.Att_Date = [Forms]![YourFormName]![cboDate]
ORDER BY S.Student_LastName & ", " & S.Student_FirstName

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Finally, after inserting the date/studentID combinations into tbl_Attendance, I would execute a requery on the subform.