I had a little boy (first grade) who always got 14 as his answer to every problem no matter what. On the second day of school I sat down to do 3+2 with him using counters. We set out a pile of 3 and a pile of 2. I told him to count and watched in horror as he pushed the counters into a line and then counted back and forth and back and forth re-counting them until he got to 14. That was the biggest number he knew, he would have just kept going on.
I had a nun who taught us the famous quote 'Vini, vidi, vici'. I was also taught that this is where the evil 666 originated. (VI, VI, VI). It was only after an embarrassing conversation as an adult that I realized it was 'Veni, vidi vici'.
There are teachers - far too many teachers - purposely 'teaching' incorrect things such as sex education in order to further their radical and/or uninformed political-religious agenda.
I had teachers 'teach' us that certain musical progressions etc were 'incorrect'. When I pointed out multiple examples of these progressions in famous masterpieces, I was told that they were exceptions, and that they were still 'wrong/incorrect' etc.
I could go on. But my point is that given the state of teaching, it's no wonder some of the students struggle.
Oh yeah, depending on the age and experience of the student, putting a thermometer in a flame to determine its temperature is not an unreasonable thing to do. A teacher should have covered that before putting students - probably children - in front of an open flame.