Hi dumb plant
Warm welcome to our forums. Pleased you found your way here. I don't think you're being whiny at all. I think you have very valid issue with school. Everyone is different, however, systems and the processes are often geared to everyone being the same. This makes for quite a bit of frustration, not just for you, but I'm sure for many students out there.
The emptiness you feel because of being ranked against others who you think are smarter is something I've felt at times too. Yes, it can leave you with a feeling of hopelessness, worthlessness and frustration. I too left school at 15, however went on to do a degree as a mature aged student later in life. Not that I'm suggesting in any way that is a solution for you. Just sharing my story.
Have you talked with anyone about your feelings about school? For example a school guidance counsellor, close family member or trusted friend? Sometimes talking helps. There are also support services if you didn't already know. E.g. eHeadspace 1800 650 890 or webchat www.headspace.org.au/eheadspace/ or KidsHelpline 1800 551 800 or webchat www.kidshelpline.com.au/get-help/webchat-counselling.
If you like I can share another story with you about my hubby's nephew who struggled with studies at school. When he was about 12-13 his parents realised that his grades were not that good - he was bored and disinterested in the school he attended. His reading capacity was almost non existent, so they found an alternative school for him. It was a school that allowed them to use the abilities they had to develop their knowledge and understanding of the world. We attended a rock concert that he helped to set up - the stage, the environment and the performance. It was awesome. He went on to do music at university and has become an accomplished musician overseas. (Still not sure how well he can read even today). This hasn't stopped him from using his abilities though and becoming a very successful performer.
I guess I'm sharing this to let you know that you don't have to give up on yourself because of the grades at school. Is there anything that interests you in all the studies you do?