I think the problem Murillo outlined is a sign of a bad teacher.

For starters students will always learn outside of class. To expect a student will not improve between classes is naive.

Maybe the student hears new grammar or words in the target languague, or they derive them through practice and applying something else they know to the languague. (Like how sports borrow from each other).

They could also recieve that information in a leason from somebody else.

Even if you are teaching secret information all of the above is possible. 

A teacher should always be prepared for a student knowing a topic to be taught.

If a leason is boring, it is not the fault of a student knowing what is being taught, it is because the interaction is not there. Maybe something is boring because it has to be done, or is monotonous/repetitive, or the reason is not apparent, or it is done at the wrong time.

A good lesson will cover/mitigate those points. A good teacher will also know it is best to change their lesson to keep the student (and themself) engaged with the material.

A student is responsible for their learning, but they delegate a lot of that responsibility to a teacher for the hour or so they are in a lesson for.

If a student is bored they will usually quietly persist, because often if is stuff that needs to be done and they have delegated to the teacher to control the leason.

If the student does speak up, What is a teacher to do?

Refuse to teach a rude student? Change on the go? Refund the lesson?

As students we hope teachers change on the go, and be good enough to continue to teach a good lesson.

But it is difficult, teachers are people and have feelings and motives. It can be difficult out of respect for a teacher to say the lesson is not good today.

I think it should be expected students have multiple (and various) teachers.

Yes, me as a student here on Italki I use different teachers for the same language, but then there should be a communication between teacher and student in case it's more a personalised or continuous work. For instance, let's imagine that I am teaching a student from the basics and I plan a lesson because I thought it'd be new and useful for this student because in the previous session I realised he or she didn't master that specific aspect, then before our next session, he has a lesson with another teacher and he learns what I had planned for the next session, it will be a bit boring for the student.

Selene, it should be a very simple thing but what happens is that many students don't like telling their teachers that they have other teachers, they think it will sound like " a betrayal " or something. So the most reasonable thing to do if you're a student that needs more guidance in the language, like, a plan and etc, is to simply be open to the teachers and tell them what you studied with the other teacher, tell them that you would like to focus on another topic or to revise the one, but without the need of going through the detailed explanation, more like a revision and so on. 

In my case, I am learning Russian and it's a bit different to me because Russian is so difficult, any topic is difficult so if different teachers go over the same grammar, topic or etc, it's a favour and a plus to me haha. But with other languages, I tend to always update the tutor " I've learnt this so I think I'm okay with this topic, maybe we could cover another one " and bla bla. 

I think that it is much better because every teacher has his own methods, opinions and organize the learning process in his own way, so you can develop different aspects of the language.

Wow, thank you so much Marynara, Aegis, Christina and Amanda for your replies and your valuable input. I really appreciate it and find it quite helpful! I am more reassured now that I can continue learning from the two teachers that I love learning with.

Thanks again for all of your help :)