What's the shortened version of Schizophrenia? Sz or Scz?

What's the shortened version of Schizophrenia? Sz or Sch?

I'd say SCZ is probably the more common abbreviation.
As long as you use the word in full the first time you use it (in a particular answer), followed by your preferred abbreviation in brackets, then any sensible abbreviation should be fine. I wouldn't suggest just using an abbreviation and hoping the marker recognises what word you mean.