Sharjah's ruler also directed that there should be only 24 teaching classes per week for each teacher, or just under five teaching classes per day.
The ruling is unlikely to affect UK and IB schools which have the smallest teacher:student ratios across the emirates.
It will however have a considerable impact on Indian, and Pakistani schools which balance lower fees with considerably larger classroom sizes.
It is not clear how much time schools will be given to implement the directives.
The Sharjah ruler's comments were made via the Direct Line Programme broadcast on Sharjah Radio. They were then reported as an official directive on state news agency, WAM.