As I recall, deriving it required a solid grasp of special relativity and calculus. I mean, if you want to say that you did that at 16, I won't argue with you. But if you claim all students did that it seems unlikely.

Edit: Oxford University, since you are from the UK and that's reputable within the country, lists General Relativity (and hence E=MC^2's derivation) as a 3rd year course in the Physics program. So that makes it squarely in the junior year.