<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/navigation_item_home"
            android:title="@string/title_activity_home"
            android:icon="@drawable/ic_home_black_24dp"/>
    </group>

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/navigation_item_assignments"
            android:title="@string/title_activity_assignments"
            android:icon="@drawable/ic_assignment_black_24dp"/>
    </group>

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/navigation_item_subjects"
            android:title="@string/title_activity_subjects"
            android:icon="@drawable/ic_list_black_24dp"/>
    </group>

</menu>

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <group android:id="@+id/navigation_group_1" android:checkableBehavior="single">
        <item
            android:id="@+id/navigation_item_home"
            android:title="@string/title_activity_home"
            android:icon="@drawable/ic_home_black_24dp"/>
    </group>

    <group android:id="@+id/navigation_group_2" android:checkableBehavior="single">
        <item
            android:id="@+id/navigation_item_assignments"
            android:title="@string/title_activity_assignments"
            android:icon="@drawable/ic_assignment_black_24dp"/>
        <item
            android:id="@+id/navigation_item_subjects"
            android:title="@string/title_activity_subjects"
            android:icon="@drawable/ic_list_black_24dp"/>
    </group>

</menu>

Add ids to navigation groups to display the divider lines
Add ids to navigation groups to display the divider lines
