The word "medium" is essential to the meaning of the sentence? I tried "Welsh comprehensive school" and "comprehensive Welsh school" just to see and both were knocked back.
Generically Welsh is Cymreig, so a 'Welsh school is ysgol Gymreig. As explained in ine of the sets of section notes, Cymraeg refers specifically to the language, so that is why ysgol Gymraeg is translated as 'a Welsh-language/medium school'.
It is quite common, though, to see signs, etc which confuse Cymreig/Cymraeg - one we see near where we stay in north-west Wales proudly announces, in Welsh, that the local cafe sells meals made with 'fresh, Welsh-speaking local produce', but while the food is good, it has so far failed to speak to us...