HMI will pounce quickly in schools, including the independent sector, when concerns have been passed to Ministers or HMI. The first schools will be put through the mill in the autumn.
The move follows high-profile cases involving allegations of staff brutality at Oakbank School in Aberdeen, bullying at the Nicolson Institute in Stornoway and non-sexual abuse at Donaldson's School for the Deaf in Edinburgh.
Brian Wilson, the Education Minister, said the unannounced inspections "will provide a quick response where concern has been identified."
The new system will focus on pupils' welfare. But it will look at any other aspects of school life which impinge on welfare, including the quality of teaching.
Graham Donaldson, the depute senior chief inspector, said: "Problems with learning and teaching often give rise to issues affecting pupil behaviour and welfare. But in that context, we will not look at learning and teaching on anything like the scale of a normal inspection."
Welfare inspections will be in two phases. The first, unannounced, will involve gathering evidence, carrying out interviews and sitting in on lessons. The second will be a notified follow-up three weeks later, which will include discussions with the school board and parents.
The existing HMI scrutiny of children's welfare in residential schools will also be unannounced from next session.