I think it is a very brave thing for you to continue to seek treatment and verification for your thoughts and feelings. Your perseverance and judgment about the need to have someone help is an important part of your healing. While it would not be possible for me to offer you a diagnosis of any kind I’ll offer what I think fits with your description. This is more like me responding to a test in graduate school where they would explain a series of symptoms and then ask what sort of things to check and rule out.
The essence of all these is the risk of acting on these thoughts and feelings and images. What is key here is to explain to your psychologist and school counselors that these fantasies have been happening for six years, that you’ve developed a plan to murder a girl in your class. You don’t want to ignore the fact that the ideation can lead to a greater risk—and neither should they.
There is more information here, but what is more important than anything else is your determination and bravery in confronting these thoughts. Please talk to your counselors and psychologist today, and if need be show them this response.