More than a third of last year's permanently excluded pupils were barred for violence against teachers and classmates, it has been revealed.
While Wales's overall exclusion rates fell in 2007-08, the percentage of pupils expelled for more serious incidents went up.
Assaults against staff and pupils accounted for 32 per cent of permanent exclusions - 77 pupils. This has shot up 11 per cent in two years.
There were also hikes in the percentage of permanent exclusions for threatening or dangerous behaviour, substance misuse and possession or use of a weapon.
Professor Ken Reid, who chaired last year's National Behaviour and Attendance Review, was concerned that a small number of pupils were becoming more aggressive.
"This could be the tip of a worrying iceberg," he told TES Cymru. But he said the drop in permanent exclusions for low-level disruptive behaviour showed schools were getting better at managing it.
The figures, released by the Assembly government this week, showed that 241 pupils were permanently excluded in 2007-08 - 50 fewer than the previous year and almost 200 fewer than in 2005-06.
The fall has been attributed to the widespread use of managed moves, whereby parents of badly behaved pupils agree with schools and local authorities to move them to another school or have them educated at home.
But these so-called negotiated exclusions have been criticised by experts, and the practice varies widely between local authorities.
Professor Reid called for managed moves to be included in exclusion figures in future to give a more accurate picture.
Home tuition continued to be the most popular alternative for excluded pupils, although more were being transferred to other schools or pupil referral units than at any time in the past five years.