A school has no authority over children except that which may be delegated by parents (and even that must necessarily be limited). Unless the parents have agreed to it, the school has no authority to force children to clean - and whether it be school corridors, classroom windows or the children's living quarters is immaterial. In addition to the lack of expressly delegated authority, there are two issues which further embolden my objection. The first is the question of what should a school be for, to which I submit the answer must be: preparation for life as an adult, the essence of which is becoming accustomed to the condition of freedom, making choices between different values* and accepting responsibility for the consequences of these selections. To that end, students must be left to decide for themselves whether, when and how to clean. The second issue is the ineffectiveness of forcing students to clean; I strongly suspect it has absolutely no bearing whatsoever on the students' cleaning habits and preferences later in life. There will probably of course be sex differences with the girls eager to do cleaning to avoid punishment, and the boys generally not giving a rubber dub duck.
All you are accomplishing is instilling the habits and mentality of unquestioning obedience to non-consensual authorities. That sobering consideration alone should weigh far more in our minds than the frankly trivial benefits of getting the school cleaning done on the cheap.
"Finally, I’d like to point out that when you say “the standing custom within high schools”, you are being slightly misleading as the custom starts in primary school …"
OK then, in primary schools. Hire a few cleaning ladies and be done with it.
*Opportunity costs: a clean and presentable environment is one value. More time spent reading chemistry is another value (an additional half hour exercise or sleep are still other values). Time spent cleaning is less time spent reading chemistry and so on.
"Calling it slavery is hyperbole not unlike the term “wage slave” because the school doesn’t own the students nor can it punish them with death or severe harm."
I disagree and I can answer this without resort to dictionary definitions.
You imply that ownership and disposal by killing or maiming are the defining elements of slavery, but that is the error of defining something by referring to a legal status. And the legal status of slavery was different in different times and places. In the second century AD, slaves in the Roman Empire for example could not be arbitrarily put to death by their masters but had to be put on trial to determine whether they had in fact committed a crime or not. Yet they were still considered slaves. I would suggest that is because the essence or defining feature of slavery is involuntary labour. It may exist in different degrees and in differing social contexts, but it is what it is.
So for you to say that forced break time chores are not a form of slavery just because this practice is legal and slavery is not, is a bit like saying when a man forces his wife to have sex (in somewhere like India), it is not rape because forced marital sex is legal and rape is not. It's an attempt to get around the problem by definition, or more specifically, of invoking narrow aspects of a legal status as a substitute for a definition.