That the ratio of the circumference to the diameter of a circle is the same for all circles, and that it is slightly more than 3, was known to ancient Egyptian, Babylonian, Indian and Greek geometers. The earliest known approximations date from around 1900 BC; they are 25/8 (Babylonia) and 256/81 (Egypt), both within 1% of the true value. The Indian text Shatapatha Brahmana gives π as 339/108 ≈ 3.139. The Books of Kings (600 BC) appears to suggest π = 3, which is notably worse than other estimates available at the time, although the interpretation of the passage is disputed. -From Wikipedia
Cubit or no, they should have been able to better approximate the circumference, perhaps to 30 1/2 cubits, which would have been closer. Though why the Hebrews would be 1300 years behind everyone else in being able to approximate the value of pi escapes me.
Regardless, like evolution, the modern, accepted value of pi is not correct according to the Bible, so I figured you would not want it taught as well. I reckon that would put you out of a job.
Then there's all that history the Bible gets wrong. Can't teach that anymore either. And remember, the entire universe is only about 6012 years old, throwing into doubt all those astronomical calculations about the distance to other galaxies based on light years, since nothing could exist over 6012 light years away or we would never be able to see it.