But I have been told to use waxy shellac by my instructor!

One of the major reasons why waxy shellac is recommended is because less amount of lubricating oil for the pad is required thus making the burnishing process easier. But is it worth the fact that it is more susceptible to water? I think not. Moreover fresh dewaxed shellac will require very little lubricating oil. Ridding the lubricating oil that rises to the surface eventually, is a simple process and worth the time than having a finish that will ruin easily with water.