It's a funny joke, but I'd be very surprised if that held at any grade school math level.
Someone would have to be numerically illiterate (with a college degree!) to be unable to do grade school math they've been teaching for multiple years.
Year 1, possible. But eventually it rubs off on even the teacher.
An interesting thing about 3rd grade math - I was taught long division. When implementing the standard C library for the PC, I implemented the same algorithm in assembler, and then again for the floating point divide emulator.
Most people suck at math. Those who don't suck usually have many well-paying jobs available for them. There are lots of schools, so many math teachers are needed.
Put these numbers together, and you realize that there is no way to have math teachers who are actually good at math. The numbers just don't add up.
Someone would have to be numerically illiterate (with a college degree!) to be unable to do grade school math they've been teaching for multiple years.
Year 1, possible. But eventually it rubs off on even the teacher.