Students performing Tayammum due to being forbidden from using the tank water by the school administration
The Salah performed by those students through Tayammum is not valid, as water is available in the place where they live.
Tayammum is permissible in the absence of water or when a person is unable to use it. If the students concerned are prevented from using the water of the tank, they can easily get water from any other place in the town where they live.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.