Dear ;
First, you mentioned that the school has a Single Phase ” 120/240 V “, and that is not possible, because a Single Phase supply meanse that you have only 1 value 120V or 240V, and you haven't both in the same time, but if you have both in the same time that means you have ” 3 Phase + Neutral “.
So, if you have a Single Phase ” 240V “, you have the possibility that Mr. Nael Samman mentioned, but if it's ” 120V ” and you have the same possibility you should use a transformer ” 120/240V ” with the necessary power. If you use 1 of these 2 cases you should be careful because after re-connecting the cooker to be supplied by 1 phase + neutral, the nominal current of the cooker will be 3 times than his initial value.
But if the cooker can't be re-connected, the school should absolutly have ” 3 phases + neutral “.