多对多关联中，附加关联的同时更新关联中间表的字段无效

用户表、班级表、班级成员关联表如下

users
    id - integer
    name - string

iclasses
    id - integer
    name - string

iclass_user
    user_id - integer
    iclass_id - integer
    is_teacher - integer

User模型

public function iclasses()
{
    return $this->belongsToMany(Iclass::class)->withPivot('is_teacher')->withTimestamps();
}

Iclass模型

public function users(){
    return $this->belongsToMany(User::class)->withPivot('is_teacher')->withTimestamps();
}

public function students(){
    return $this->belongsToMany(User::class)->wherePivotNull('is_teacher')->withPivot('is_teacher')->withTimestamps();
}

public function teachers(){
    return $this->belongsToMany(User::class)->wherePivotNotNull('is_teacher')->withPivot('is_teacher')->withTimestamps();
}

表单提交

以下为Dcat Admin数据表单片段代码，参数释义参照：事件以及表单响应《Dcat Admin 中文文档》

$form->saved(function (Form $form, $result){
    if ($result && !empty($form->input('iclasses'))){
        $iclass_ids = explode(",", $form->input('iclasses'));
        $relations = [];
        foreach ($iclass_ids as $iclass_id){
            $relations[$iclass_id] = [
                'is_teacher' => 1
            ];
        }
        $form->model()->iclasses()->attach($relations);
        //$form->model()->iclasses()->sync($relations); //同样无效
    }
});

以上代码，中间关联关系表数据插入正常，唯独is_teacher字段写不进去值。
初步学习Laravel，文档理解的不够深刻，还在持续学习中，请各路大仙指点，不胜感激！