Student unions tend to focus on all sorts of other issues, I wouldn't trust them to handle cases like this.
The only way to reliably prevent the use of AI tools without punishing innocent students is to monitor the students while they work.
Schools can either do that by having essays be written on premise, either by hand or by using computers managed by the school.
But students that are worried that they will be targeted can also do this themselves, by setting up their phone to film them while working.
And if they do this, and the teacher tries to punish someone who can prove they wrote the essay themselves, either the teacher or the school should hopefully learn that such tools can't be trusted.
It's also the case that even pre-Web and certainly pre-LLMs, different schools and even departments within schools had different rules about working with other students on problem sets. In some cases, that was pretty much the norm, in others strictly verboten.
The only way to reliably prevent the use of AI tools without punishing innocent students is to monitor the students while they work.
Schools can either do that by having essays be written on premise, either by hand or by using computers managed by the school.
But students that are worried that they will be targeted can also do this themselves, by setting up their phone to film them while working.
And if they do this, and the teacher tries to punish someone who can prove they wrote the essay themselves, either the teacher or the school should hopefully learn that such tools can't be trusted.