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I teach as a private tutor and most of my students are prohibited to use techniques and/or language constructs that they haven't been taught!

There's a really broad variety of stuff that is prohibited like foreach-loops, switch-statements, functions/methods, etc.

What do you think is the benefit of this rule or why do most schools and universities have such a rule?

Rules like this are generally instituted because the teacher is attempting to teach a concept made moot by one of these constructs. For example, as a teacher, if you're teaching bitwise operators and ask students to implement absolute value, it simply makes no sense to permit the students to use whatever library function does absolute value. You're teaching bitwise operators, and the fact that they know of the library function doesn't help them learn bitwise operators.

I believe that because you're asking here, the reason for the prohibition isn't as obvious as in my example, but my guess (without knowing the specifics of the situation) is that the prohibited constructs are not what are being taught so, for that course, the students knowledge of a more advanced concept is irrelevant to the subject matter being taught.

I think that the answer of thesecretmaster is correct but let me add a bit of advice to an instructor who would do this. Just as you wonder yourself, the rule doesn't seem to make a lot of sense and it won't make sense to students either. It may cause resentment.

So, if an instructor wants to use a rule like this then, I think that a general rule agains using things not yet taught is foolish. However, you can achieve the same result by carefully stating the problem you want solved. So, for example, following thesecretmaster, instead of asking for the students to compute the absolute value ask them to compute the absolute value using only the bitwise operators. In other words, put the specific restriction into the question itself.

This brings up an important teaching and learning technique called "creativity under constraint". Some wood working artists, for example, use only hand tools, forgoing power tools. Furniture makers I've known do this. The restricted set of tools forces them to make better use of the tools that the do use, becoming more skilled in the process. But, to impose it on someone else requires that you make it clear why it is a good idea to do this.

I've discussed Creativity Under Constraint here in the past in other contexts. See this post and this other one.

Where it stems from is, of course, because the lab is not the thing that the instructors want solved. After all, the lab problem is not an unsolved problem, and it will only be unique (if at all) in some surface way. This is the source of the feeling that people have that the restrictions are unreasonable: they feel like solving the lab is somehow the purpose of the assignment, and therefore any high quality solution is to be praised.

So if solving the problem isn't the deeper purpose of a lab, then what is? Typically, we are teaching an algorithm or a data structure. The lab is conjured as something that lends itself to that algorithm or data structure, and that is the goal of the instruction. I want to engender mastery of linked lists, or stack management, or two-dimensional arrays, or memory management, or... whatever the focus is.

The lab problem itself is entirely secondary, even if it does not in any way feel like this to the student. The lab problem is simply meant to provide a rich environment to play around with the learning target, and to gain some measure of experience wrestling with it.

The problem that we run into, then, is that there is no problem that cannot be solved in many ways. I can search as hard as I might for a problem that would be much, much harder to solve in a manner perpendicular to the purpose of the lab, and sometimes I will have some success. But sometimes I will not, because sometimes no such problem exists.

There exists no problem that can be solved with a linked list that cannot also be solved with an arraylist. I could provide starter code to try to force my approach, but that simultaneously increases the difficulty of creating the lab while decreasing the thought that must go into starting it.

A blanked ban, such as "you may use no bang operators in your Racket lab" is an imperfect solution, but all of the solutions are imperfect, and sometimes a ban feels like the least of the evils.

This is not a blanket defense; such band can be careless, or needless, or clumsily done. I try hard to avoid them in my own instruction, but I don't always succeed. If it seems unfair to restrict my students' approach, it is also unfair to my students if they don't delve far enough into the course material because they originally thought of a different solution, and they just stuck with it. That cheats them of the chance to learn the material in the course, which they may well need in their next course, or later on in their life.