When I was student teaching, I had one who was JUST on the verge of passing (thanks to the incredible mercy of the primary teacher). All he needed to do was turn in a worksheet that he finished in class. I know that he finished it because I watched him and helped him do it.
All he had to do was give it to the teacher. But, in his mind, that would mean that she had won. So he refused to turn it in. I left the school before the end of the semester, but I would bet money that he failed the class.
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.