You can't just demand kids sit, listen and learn the minute the walk onto school grounds.
But SLT could run induction sessions - perhaps as long as a week -
for children entering secondary school, to explain to them why good learning behaviour is important for their futures and for the futures of the other children in the class, and to model and let them practise the expected behavior. SLT could set up a centralised system to enforce discipline, taking responsibility for it themselves, so that teachers don't have that piled on top of everything else they have to do. And for children with serious problems, SLT could provide a nurture unit, which many schools do, to help them gradually integrate into the school and its routines, and provide a refuge when things get difficult. This is the model they use in Finland, for example. Some schools already do these things.
And what these kids need more then anything is consistency, security and stability which they aren't going to get when they don't know who is covering the next lesson because there is not enough teachers and no money in the budget to hire new ones
But it's already been pointed out that it's not just a question of money. It's starting to be, but the recruitment crisis has been going on for a while. One of the reasons for the recruitment crisis is disruptive behaviour, and the teacher-bashing belief by some SLT that 'if you make the lesson engaging enough, the children will behave' that pushes teachers into edutainment and dumbed-down expectations. Another is the expectation that teachers can achieve miracles for children's learning in circumstances where large amounts of learning time are lost to low level disruption. Disruption inevitably has adverse effects on learning, which contributes to the demand by SLT that teachers work harder to compensate.
Complaining about parents who argue that children should be expected to concentrate and work seems to me to be a very strange approach to solving the recruitment crisis.
And while having the same teacher undoubtedly is important for stability and security for children with problems, so is a classroom that isn't chaotic and constantly disrupted by poor behaviour.