The biggest issue we've been finding with child in son's class, isn't the school, it's the parents.
They have been in complete denial about issues and think their son is just 'a rascal' rather than some sort of issue.
We've deliberately avoided engaging with them directly and made a point of saying we'd do things through school until things calmed down with our son, rather than pinning anything on them.
The thing with talking to other parents directly is you just don't know how they will react. In the worst cases they could become aggressive and spark an incident with you. In a lesser scenario they might go massively on the defensive and cause problems for you or your child in other ways. It's just not worth it, because a direct intervention - unless you know someone really well - is likely to be taken as an insult, criticism or other form of personal attack on their parental skills even if what you say is well intentioned.
Going through the school and letting them know of incidents is better as they will have a fuller picture and be able to take the emotional side of things better because the focus is on helping their child do better rather than saying in someway 'your child is causing issues'. The former is a positive framing and the latter is a negative framing.
If the other parents are in denial, it's a slog and an uphill battle however it goes. Having the support of school is useful. ATM we are feeling frustrated and quite powerless as it's only edging forward slowly - even if that's to get further interventions for the other child - but we also understand going through due process is the best option to facilitate and that involves closely working with school.
Honestly I wish the system wasn't so overstretched and imputant but we are conscious that there are ways which it could explode in our faces even worse if we start being confrontational.