I am not 100% sure if this is the case in England but in the part of the UK where I live, schools can directly refer to CAMHS for things like ADHD and ASD assessments. So I would speak to you teacher in the first instance to see if that is something they may be able to do.
If your school is not able to refer to CAMHS, you would need to speak to your GP and say that a teacher has raised this concern and ask them to refer instead. Be aware that an ADHD assessment will require getting information from your parents or other evidence of symptoms early on in your life so if your parents will not be willing to engage, you might want to look for things like old school reports that might add evidence.
I would prepare for a very long wait though, this is not something that will be sorted in a couple of months. Appreciate this is not ideal, but the other option may be to wait until you are at uni and see if your uni may be able to refer for an assessment (I have seen people mention this but I have no idea how this route works).