Fewer than half of all secondary pupils permanently excluded in 2005-6 were given home tuition or transferred to another school, new Assembly figures reveal.
They follow concerns that children at risk of being thrown out of schools should receive early professional help to steer them away from a life of crime.
More than a quarter of the 451 pupils excluded from schools indefinitely in 2005-6 were tutored at home, according to the figures. Around 17 per cent were sent to other mainstream schools and a further 2 per cent were re-instated at the schools after appeal.
The fate of the remainder - around 250 pupils - has not been recorded.
However, figures recently released from the Youth Justice Board show that 15 per cent of all school-age prisoners in England and Wales last year were excluded pupils.
Jane Davidson, minister for education, lifelong learning and skills, said last year: "There is much evidence telling us how important education is in keeping children and young people from crime."
But unions claim a lack of feedback from external agencies in some local authority areas has meant they are not getting information about troubled pupils. The figures show the number of secondary pupils excluded from Welsh schools for five days or fewer has almost trebled since 2000.
There were 601 more primary pupils sent home in 2005-6 on short-term exclusions since 19992000 for bad behaviour. Boys were the biggest culprits, accounting for seven out of 10 permanent exclusions from Welsh schools in 2005-6.
But the number of girls thrown out of schools for bad behaviour also went up. Rule-breaking (27 per cent) followed by threatening and dangerous behaviour (12 per cent) were the top two reasons.