I have been looking at Lancelot Hogben's Mathematics for the Million (first published in 1936).
In the chapter on calculus he says that the second derivative $\displaystyle \frac{d^2y}{dx^2}$ is
pronounced dee-two-wy-by-dee-eks-two.
The final "two" surprised me as I was taught to say "squared" and I have never heard it pronounced differently.
Assuming that it's not a typo and that Prof Hogben wasn't mistaken, did people really say "two" in those days and does anyone say "two" these days?
I suppose that for higher derivatives just saying the number rather than the power it represents may be easier.
According to this post there are several ways to pronounce $\frac {d^2y}{dx^2}$, including:
1.) "dee squared wai over dee eks squared"
2.) "the second derivative of y with respect to x"
3.) "dee two wy by dee eks two"