Hi and welcome to Pianostreet.
Have you asked your teacher about playing more challenging music, other than the Fantasie Impromptu? What was your teachers response?
Did your teacher give a reason for not learning the Fantasie Impromptu?
Does your teacher have other students that play music at the difficulty level of the Fantasie Impromptu?
Six years in music school is a decent amount of experience playing and learning. Although some of your other repertoire is not at the same difficulty as the Fantasie Impromptu, you are not far from it.
What you can do is try out small sections of the Fantasie Impromptu. If it is too overwhelming, wait a bit more to build up your technique. It would be preferable that you study this piece with the guidance of a teacher, since it is a stretch from the difficulty of your current repertoire.
Some teachers like to stick to music that is familiar to them and do not like it much when students get adventurous and start exploring. Other teachers just specialize in teaching a certain range of levels, for example beginner and intermediate, but not advanced. Although from what you describe, it sounds like your teacher is being dismissive, which is unfortunate because they should be grateful that you are showing interest in pursuing your studies with more advanced music. At the very least they should be giving you a detailed response on why you should or should not study the Fantasie Impromptu at this time.
Topic: fantasie impertru