That's ridiculous. Math, science, vocabulary, history, and reading comprehension can all be tested effectively, with little risk of "gaming" it.
(at least at the level of a tpyical high scholl curriculum)
It is difficult to game your way to an A without knowing a fair amount of the material, but it's definitely possible to get a passing grade and possibly as high as a B. And students are often very eager to try to claw as much partial credit on a test as possible.
My wife teaches at a 4-year state school and tells me that one of her students did just that, she looked for plausible keywords but then didn't know what to do next, this was chemistry, not math.
Reading comprehension is harder to game.
> she looked for plausible keywords but then didn't know what to do next
That doesn't sound like she arrived at the right result.