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That’s not what they said. Pressure isn’t the same as a requirement. The only (legally permitted) consequences for refusing to say the pledge are that some people might be mad at you.

If that was a public school, that’s actually illegal. There’s a giant stack of court precedent about it, including at least one SCOTUS ruling – you cannot punish or treat a student differently for refusing to participate in compelled political speech, and the Pledge specifically has been tested in this regard and you can’t force that. Some parent (or recent graduate who has turned 18) needs to sue that school system ASAP.

It’s a thing with some teachers in some places. The quality of education in the US is hugely variable, because standards and curriculum are largely left up to local school systems with widely different funding and priorities.

That was a short answer that turned out to be mostly wrong. The longer answer is “if you don’t understand how this works, you won’t have the intuition to notice when you get absurd results from the calculator”. If you don’t have that intuition, then when you inevitably make a small usage mistake on a calculator (or in matlab or wolfram alpha or whatever), you’ll end up not realizing that you got a clearly wrong answer.