Hi Deena.

I'll assume you mean US gallons and short tons as used in the United States.

First, the dimensions of the classroom can be multiplied together to give you the volume of the classroom in cubic feet.

Next, you need to convert from cubic feet to gallons. You need a conversion factor. When I ask www.WolframAlpha.com to convert "1 cubic foot in US gallons", it tells me the conversion is 7.481. So by
multiplying the volume in cubic feet by this, I will know the number of gallons.

For the last part, you are converting from a volume (of water) to its weight. When I ask www.WolframAlpha.com for the "density of water in short tons per cubic foot", it tells me that the conversion factor is 0.03121. So again, I can multiply the volume in cubic feet by this to find the number of tons.

Try it.

Hope this helps,
Stephen La Rocque