A report by the Labour Research Department for Unison said support staff “bear the brunt of behaviour issues, including violence”.
Since the pandemic, they have faced more poor behaviour and greater numbers of pupils with complex needs, Unison said.
Rosie Lewis, Unison Cymru’s head of schools, said: “Often our members are suddenly expected to undertake roles administering medication and, more alarmingly, quite personal intimate interventions such as inserting catheters.
“These are staff who work long hours, often helping out, going on residentials.
“At Christmas time, helping with school plays, getting costumes ready, all of this on top of their daily tasks and they don’t get any extra pay for that.”
The report found that many teaching assistants were in single-income households and a significant proportion were reliant on benefits.
It said school support staff, 90% of which are women, are often “the first to go” when schools cut jobs because of tight budgets.
At the same time, schools struggle to recruit because pay rates “are not competitive compared to jobs in hospitality and retail” and there is a high turnover.