Correct me if I am wrong but wouldn't a 12 year old be at least in Junior 6 or even Form 1? From junior 6 kids move from Chiswich to St Martins College and those uniforms don't look too bad: http://www.chsmck.com/smc/content.aspx?id=40218
When we were looking at schools in Malta I was under the impression that St Catherines was a girls only school, but I just noticed that they have a new website and that they are a co-ed school now. When I contacted St Catherines they were not open to our kids starting mid-year at all.
We initially liked Newark College (I assume Newark School is their primary school) too but we were not able to actually find the school (!?!) and after visiting St Martins we didn't see the need anymore.
If uniforms is a major concern, I would visit the uniform store used by most independent schools in Malta. It is on the second floor of a computer store called Klikk which is easy to find using Google.
Update: the uniform store I mentioned is called ID Schoolgear: http://idmalta.com/schoolgear/
or search for ID School Gear on the maltese yellow pages website http://www.yellowpages.com.mt/Malta-Business/Id-School-Gear-In-Malta-Gozo.aspx