Schools are failing to spot ADHD and autism, which could be contributing to a rise in exclusions, an alternative education provider has said.
Kelly Rowlands, who runs schools for excluded children, said seven out of nine of her pupils arrived with undiagnosed neurodiversity issues.
The rate of permanent exclusions in Wales almost doubled between 2014 and 2017 to four in every 10,000 pupils.
The Welsh Government said permanent exclusion was a last resort.
"The decision to exclude a child is a last resort and never taken lightly," said Eithne Hughes, director of the Association of School and College Leaders Cymru.
The assembly's education committee said in January 2019 there were 2,286 pupils in Wales were receiving education other than at school (EOTAS), many of whom had either been excluded from mainstream education or were at risk of exclusion.
The latest figures available show in 2016-17 there was a significant increase in the number of permanent exclusions compared with the previous year, up from 109 to 165.
Ms Rowlands, senior schools manager for ACT, which runs schools and training for excluded children in Cardiff and Caerphilly, said too few resources were being used to understand why a child might be behaving badly before the "exclusion cycle" begins.
She said the overall number of pupils her service had been asked to take on had tripled in three years.
"I think schools are being put under much more pressure in terms of their outcomes and their achievement data," she said.
"If they do have a disruptive learner in the classroom it's much easier to remove them and deal with the masses than it is to focus the time on that one person."
She called for a "stricter screening" of children before they start secondary school to try to identify any additional needs that may have been missed.
Riley, 15, was excluded following an assault after starting secondary school.
"I was always being naughty, I punched my head teacher, chucked a chair at him," he said.
"I was getting angry all the time - with the students, with the teachers. I just didn't like the school."
Amanda Kirby from the University of South Wales is conducting research on a link between school exclusion and additional learning needs.
She said children with special educational needs were "seven times more likely" to be excluded than those without.
"What we've seen in Wales is that exclusions have been on the increase," she said.
"I think that even though they're relatively rare, if you look at the total population, we are still recognising that in both Wales and England we're seeing a general increase. That's something we need to be alerted to..."
A Welsh Government spokesman said: "Our guidance makes clear that permanent exclusion should be a last resort and should only be used where all other strategies for supporting the pupil have been exhausted."
He added legislation had been introduced to ensure behavioural issues were dealt with earlier and more effectively.
Ms Hughes said support for children with behavioural problems had been "hampered by severe funding pressures".
"Despite their best efforts, it is occasionally necessary to exclude a pupil to ensure that other pupils are able to learn in a safe and orderly environment, and also because the pupil in question needs more support than a mainstream school can provide.
"These are difficult and complex decisions over which schools, governing bodies and local authorities follow detailed and extensive guidance," she said.
Watch more on Wales Live, at 22:30 GMT on Wednesday on BBC One Wales, and on the BBC iPlayer.