how to make role menu in laravel

I need to make role menu in laravel , i have make in User model this funcion

public function isTeacher() {
   return $this->where('role', '3')->exists();
}

and at the app.blade making this :

@if(Auth::user()->isTeacher())


        <li class=" btn">
                <a class="menu"  href="/">الرئيسية</a>
            </li>
            <li class=" btn">
                <a class="menu"  href="/">تقييم الأطفال</a>
            </li>
            <li class=" btn">
                <a class="menu" href="/">تسجيل طفل جديد</a>
            </li>
            <li class=" btn">
                <a class="menu" href="/indexTable">إدخال جدول الانشطة</a>
            </li>
            <li class=" btn">
                <a class="menu" href="/">مراجعة جدول الانشطة</a>
            </li>
        @endif
     @if (Auth::guest()) 

            <li class=" btn">
            <a href="/register" class="log"> تسجيل ولي أمر جديد </a>
                </li>
@endif

But it's giving me that error :

Call to a member function isTeacher() on a non-object

via Dina Shaldoum