mysql_fetch_assoc($result) problems

Short summary for @cane23: the fetch loop is blank because the query is returning no rows (so there is nothing for mysql_fetch_assoc to return). Two likely root causes in the posted script are (1) the SQL string is malformed and mixes the script's DB-login variables with the intended form input, and (2) the script prints numeric indexes while using an associative fetch. @kireol spotted the malformed SQL and @Derice pointed at associative vs numeric indexing — both are relevant.

Quick, practical checklist to debug and fix the immediate problem:

• Confirm which variables hold the database credentials and which hold the user input; use distinct names (for example, $dbUser / $dbPass vs $formUser / $formPass) so values do not get mixed.
• Output the complete constructed SQL during development to verify it looks as expected, then run that same query directly in the database client to see whether it returns rows.
• After a query, check for errors from the database driver (for the old extension use mysql_error(), for mysqli/PDO use the appropriate error method).
• Check the result row count before looping (so you know whether zero rows or a fetch error).
• Inspect the returned row keys (use functions such as array_keys or var_dump on one fetched row) so you know the exact column names to use when reading values.

Prefer a modern, secure approach instead of the old mysql_* extension: use prepared statements (mysqli or PDO) and store passwords with password_hash/password_verify. A concise PDO-style pattern (illustrative) is shown below.

$pdo = new PDO($dsn, $dbUser, $dbPass);
$stmt = $pdo->prepare('SELECT password_hash FROM students WHERE id_no = ?');
$stmt->execute([$submittedId]);
$row = $stmt->fetch(PDO::FETCH_ASSOC);
if ($row && password_verify($submittedPass, $row['password_hash'])) {
    // authenticated
}

For migration and secure-password guidance see the PHP docs on PDO (PDO manual) and on password handling (password_hash, password_verify).