Dropdown Box is not populating from MYSQL

Dropdown Box is not populating from MYSQL

I have problem in populating dropdown from mysql in php form. Database
Connection is just fine. But dropdown returns empty. I have two tables.
(id, name, age, etc) and admissions (admissionid, year) Each admission can
have no of students. but each student can have only one admission year. I
am trying this code:
<?php
ob_start();
session_start();
include ('database_connection.php');
$Studentid = $_SESSION['Studentid'];
$sql = "SELECT Admissionid FROM admissions";
$result = mysqli_query($sql);
echo $result;
$dropdown = "<select name='admission_year'>";
while($row = mysqli_fetch_assoc($result)) {
$dropdown .= "\r\n<option
value='{$row['Admissionid']}'>{$row['Admissionid']} </option>";
}
$dropdown .= "\r\n</select>";
echo $dropdown;
?>