My understanding is that to teach in any state school (including a school for pupils with MLD, SLD and PMLD) you have to have QTS. Special schools do not have any different requirements to mainstream in that regard. They usually require experience with pupils with SLD or PMLD as an additional skill. I do know that our special school would not employ someone as a teacher without QTS, although they may employ someone as an unqualified teacher or a teaching assistant with QTLS. Jobs are scarce though and we're likely to have to loose some staff (the head says through "natural wasteage... what an awful term) due to buget cuts and the shiny new PFI school costs.