Try this:-
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Online Student Portal</title>
</head>
<body>
<form action="">
<input type="button" value="Add Students" onclick="window.location.href='Students.html';"/>
<input type="button" value="Add Courses" onclick="window.location.href='Courses.html';"/>
<input type="button" value="Student Payments" onclick="window.location.href='Payment.html';"/>
</form>
</body>
</html>