SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add laravel

Question

I am getting this error:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (prj_tvto.activities, CONSTRAINT activities_school_id_foreign FOREIGN KEY (school_id) REFERENCES schools (id)) (SQL: insert into activities (school_id, cluster_id, group_id, updated_at, created_at) values (0, 2, 3, 2018-08-14 05:02:28, 2018-08-14 05:02:28))

In Controller

public function store(Request $request, SchoolsList $schoolsList)
{
    if($request->ajax()) {
        $activity = new Activity();
        $activity->school_id = intval($request->school_id);
        $activity->cluster_id = $request->cluster_id;
        $activity->group_id = $request->group_id;
        $activity->save();
        return response()->json(['data_activity' => $request->all(), 'id' => $activity->id]);
    }
}

And Ajax

        var school_id = {school_id: data_school.id} ;
        console.trace(school_id);

I am using laravel 5.4