I personally don't know much about Scuba Shack, in particular but I know a few good setups in Malta. I lived there for 18 Years and I still visit. There is big competition in diving Schools I cannot imagine any bad once.
It all depends what sort of diving you do, where you are staying - most hotels will set you up, etc.
I usually use Aqua Bubbles down in the south end of Malta. Very close to a number of wrecks (inc WW2)& Ray fills nitrox too. I was always treated very well and the prices were very reasonable. In Gozo I used St Andrew's and for nitrox in Gozo I used Dwejra Divers. If you go dont miss diving in Gozo I think its nicer
. If you are more into tech diving (trimix and RB), use Strand.
I will be going there this May.
Take care
Lawrence