Wanting To Drop A Subject

Hi, I'm doing maths, physics, electronics and economics however, I am barely getting E's in my economics. My other subjects are going well with a B in physics, C in maths and an A in electronics. I want to drop economics as I feel that I could spend the extra time on the other subjects and to avoid putting a U for economics if I fail. However my Head of year says that I can't as they are not convinced that I will do as well in my other subjects and not pass them. Just wanted some advice on how to proceed with this.