Word Study - 'sc' or 'sk' ?

Here's an example of some of the word study work we do in Room 4.

We've been discussing if we use an 'sk' or an 'sc' in words and how do we know which one to choose when they both can sound the same.  We began to write some 'sc' sounding words on our whiteboard and came up with a theory.  We were then able to organised these words into columns to show our thinking.  This is what we came up with.