Django -表单无效但没有错误

Model:

模型:

class Session(models.Model):
    tutor = models.ForeignKey(User)
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()
    def __unicode__(self):
        return u'%s - %s' % (self.tutor, self.start_time)
class Session(models.Model):