Cylinder (astigmatism) >1.00 diopter, especially oblique, degrades reading performance. Amounts >2.00 diopters induce ‘lazy eye’ and cause sometimes severe medical and pseudo-psychiatric disorders.
The ciliary body doesn’t care about spherical equivalent. (you can quote me on that)
Now that’s something your school principal DOESN’T know.
You can find data to back this up through many other sources. Search for ‘VT Proof’ and ‘Proof’ on this site and LearningManagement.ca.