Your teacher's pension is taxed for administrative reasons only. HMRC calculate your overall tax liability and, for simplicity, take what you owe from just one of your income sources. So your teacher's pension takes the total hit of your liability so that your state pension can be left alone. It's all fair enough. I also get letters telling me that I have underpaid. My affairs are incredibly simple and I can see no excuse for the HMRC to get it wrong, yet they do.