Well, first, need to read you the riot act -- this is 2014, you shouldn't be writing new programs using mysql_ functions, since we've been told for EIGHT YEARS to stop doing that, and for almost three years now there have been giant red warning boxes in the manual telling you not to do that!
Second, blindly dumping $_POST values into a query string is BEGGING to get hacked.
Now that's the initial bad news, the good news is if you switch to mysqli or PDO like a good little doobie, "prepared queries" will mean you CAN dump $_POST into a bind or execute array; and the prepare/execute model is designed for EXACTLY the type of thing you are doing iterating your values.
Now, that said... Your form has problems. on top of the fictional border attribute value, attributes like border, align, width that have no business in any HTML written after 1997, and missing/incomplete closures -- there's the real root of your problem, -- name attributes that don't line up.
<td align="center"><?php echo '<input type="hidden" name="att" value="0"/>';?><?php echo '<input type="checkbox" checked="checked" name="att" value="1" />'; ?></td>
On top of the mutliple echo for nothing, you realize that the first att will be att, the second one att, on the next loop it will be 2 and 3, so they will NOT correspond to "rollno" 1:1 when you try to loop through them.
I'd suggest putting $rollno as the index inside ATT... though, is rollno really appropriate? Wouldn't the ID make more sense? Well, maybe not... not sure I follow what it is you are even trying to here as it makes no real sense.
Guessing WILDLY, let's start by fixing the markup:
<form action="attendinsert.php" method="post">
<th scope="col">Student Name</th>
<th scope="col">Student Roll.No</th>
/* assumes $db is a connected PDO object */
$statement = $db->query('
SELECT id, name, rollno FROM registered_members
while ($row = $statement->fetch) echo '
<th scope="row">', $row['id'], '</th>
<td>', $row['name'], '</td>
<td>', $row['rollno'], '</td>
name="att[', $row['rollno'], ']"
<input type="submit" class="submit" value="submit" />
Then, let's fix the handler... no, wait... what the ? Ok, I'm just not getting what it is you are even trying to do here.