"A Welsh school"
This is (I think) the first example of a mutation being used in the course, which is interesting in itself.
The rule here is that an adjective describing a feminine noun (ysgol, in this case) will take a soft mutation, so the C softens to a G. I expect there'll be more detail on these as the course progresses.
Welsh has mutations. This is an example of a 'soft mutation' (C -> G). For a long list of rules regarding mutations see: http://www.bbc.co.uk/wales/learnwelsh/pdf/welshgrammar_mutations.pdf