If a teacher wants to represent a mathematical system which is used to convert the weight of water to the mass of water he surely has to take idea of acceleration of gravity as a basis when making a description. Teacher will point that weight is mass multiplied by acceleration of gravity which is equal to 9.81 m/s2. And, of course he will mention the definitions of mass ( the amount of matter) and weight (which affected by the pull of gravity). These are the most essential points.
Hope that helps.