First let's clean up that first function a bit; you don't need those variables, and even what you're passing to the view looks overboard.

public function show($id) {
    $class = Subject::where('id', $id)->first();

    if($class->user_id == Auth::id()) {
        return View::make('subjects.show', array(
                             'class_name' => $class->subject_name,
                             'class_id' => $class->id,
                             'title' => $class->subject_name, 
                             'userFirstname' => Auth::User()->firstname,
                             'userLastname' => Auth::User()->lastname,
                             'userEmail' => Auth::User()->emailaddress,
                             'id' => Auth::id(),
                             'user' => User::find('id'),
                             )); 
    } else {
        return Redirect::to('dash');
    }
}

And to get the right id you need to pass it to create() like so:

{{ HTML::link('http://test.com/create/'.$class_id) }}

Then your create() becomes create($id).