Unfair maths gradesWatch
Try to firstly address your college about this and why they may assess their grades differently, maybe you'll get a bit of clarity there. I would try to maybe do more past papers for teachers to mark so you can prove to them you are better than a D. Plus its very weird for them to not follow your exam board's grade boundaries.
My college has been making their own grades to assess my a level maths. Working at a consistent 55% of full marks i am working at a D, however looking at the 2018 and 2019 grade boundaries i should be working at a grade B. I'm really worried that I might not get a C or higher as my final grade as a result
If you wish to understand their judgment I suggest you request a meeting with the teacher to discuss this.
They are the only people who can offer any guidance on this.
His school royally stuffed him up!
Schools are merely enacting the decisions - if you feel like complaining to someone, try Gavin Williamson.