A lot of principles in teaching and learning are taught, tested, re-proven to work, and eventually found their applications. However, there are those that exist but are better left unsaid.

Examples of Some Principles that are are taught:

In Chemistry

"Like dissolves like" is a fundamental  truth in dissolution of substances.  This means that a polar solute dissolves in polar solvent, and non-polar solute dissolves in non-polar solvent. Thus, oil, being non-polar, will not dissolve in water (a polar solvent), but will dissolve in kerosene (a non-polar solvent)..

In Assessment and Evaluation

"Assessment should not be used as punishment".  This means that assessment of learning is used to gauge what the students learn and not to punish a student.  By this,  it is in contradiction to this principle when the teacher intentionally gives a quiz at the  first hour of the class just to punish students who come in late.

There are many more and you can cite many of  them. 

How about  something (a principle) that is hidden? One that exist, but teachers will never tell. Well, here it is,

"DON'T MAKE IT HARD FOR ME, SO I WON'T MAKE IT HARD FOR YOU"

In christian nations, good values are taught this includes teaching students that vengeance is not good for "vengeance is not ours, but God's"

In spite of that biblical advice, in reality that principle (Don't make it hard for me, so I won't make it hard for you) does exist.  Some teachers struggle to avoid it, some just let it flow naturally, and some oppose it .  Well, it all depends on the teacher. After all, teachers can be tricky on this.

It does exist. 
Here, are some examples:

When your teacher gives you an instruction on assignment and you submitted the wrong one or you simply did not follow instruction, it is likely that you will either fail  on that activity or you will get a big minus on your grade.

In a an examination where essay type questions are given: When your handwriting is bad and the checker finds it hard to read your answer, again it is likely that you will either fail or you will get a big minus.

When you are being  an unruly student and you make it hard for your teacher to deliver his or her teaching, watch out during the computation of your grades- a borderline may be tilted towards the failing side of the scale.

THE TEACHER WILL NEVER TELL.

As you may discover, this hidden principle can find application in other situations. But then again remember, "Vengeance is not ours, it's God's"