I have a question for you
I know some people who are musicians, they even teach music to kids… but they never studied music without any certifications. It's a sthereotype maybe if someone is raised with classical music, and may it's even because some teachers are a bit distant… they always said to me that a good musicians, even if a musician is emotional, he/she must have technique. They said, for example, that pop musicians do things like… simply, not caring about anything. I, myself, know some musicians who are really excellent, but NEVER studied music in academies or institutions
I think my classical music teachers were not right, because it's not true that if you don't study in institution, you're bad
Is it just a sthereotype institutions have to let students go ahead?
If you are asking whether studying in music school is necessary to be a “good” musician, the answer is certainly a no. But “musician” is a very general term; people who go to “music schools”, in general it means they go there to study western European classical music. There are great musicians out there who have never studied in music schools, that is true to many genre other than classical music. There are certainly talented people who started learning instruments at very young age, and have great teachers/mentors to educate them into well rounded musicians. There's always exceptions.
And as someone who has studied classical music in conservatories, I agree with your classical music teachers. A musician must have high level technique in order to express art. It is like learning a language, the better one is with words, the better one can communicate one's emotion. Everyone has emotion, but it's useless when one cannot express in ways that others can understand.
In music school, students learn about music history and theory because these knowledge help one to understand composers' intentions to make informed artistic decisions. That is, a musician should be able to tell you and defend why they are performing a piece in such way, rather than “because I feel like it”.
Students also study with major teachers, coaches and conductors, who are established musicians themselves. Mentor-ship is very valuable in connecting all the different things one learns in a music school. It is harder to get that many mentors when one is only learning on his or her own.
Students learn to work and collaborate with each others professionally. That is, how to rehearse efficiently for the best result. Music schools provide years of opportunities in variety of individual and ensemble training, which an individual cannot easily obtain on his or her own.
Therefore graduates of music schools are regarded as more informed musicians. “Good” & “bad” are extreme (dangerous) words in arts, because opinions in arts are always subjective. But technique is objective, so you can say this person has good or bad technique. Again, if one has bad technique, one cannot express well.
It is unfortunately difficult to convey the impact of going to a music school, if you were not yourself experienced in that environment. Academics and institutions are not perfect, but they only focus on a very specialized field - western European classical music, not “music” as a general term. So, if you want to argue that you personal know “good” musicians who NEVER studied in music school, you have to be specific and what is your definition of “good” before the argument could be taken seriously.