Saturday/evening detentions, segregation from class, and expulsion. You failed to come up to standard, in the work set as punishment you repeated time and time again till acceptable.
Traditional old fashion standards as we call them today, didn’t decline and fall out of fashion as with the state school, they were retained and still maintained today. Parents pay highly for such placements they expect their children to be disciplined, and reach high standards, they support their choice of schools, take an interest in their childrens education, and expect the same standards at home.
When there was national service it didn’t take long for a group of mismatched lads to be turned around in the very short time of basic training. Private schools are very similar, pupils are institutionalized very quickly, the high standards required become second nature and follow through life. Yes, there will be the small minority that don’t comply, as in every walk of life.