If they are the school’s I would simply not give them to that student anymore, for very obvious reasons that every 9yo should understand. Let him use his own.

Is the act itself disturbing the class or his own ability to concentrate? If not, I do not see any further problem.

Haven’t we all chewed on pencils to concentrate?

Talk to parents, get their written approval to put bitter apple spray on the erasers.

Something similar to the spray they use on animals after a surgery? It’s safe for consumption but it tastes god-awful to stop the animal licking the wound.

Sounds like it could be a stim thing - impulsive, you say? Any chance there’s (undiagnosed?) ASD there? The mentions of bitter spray reminded me of when my mother tried that to get me to stop biting my nails. I just stopped using my lips and tongue, and only used my teeth…

Anyway, if it’s a stimulation thing, maybe finding an alternative would be easier than getting him to stop entirely.

Some pharmacies sell a bitter liquid (no idea how it’s called) intended to to put on children’s fingernails to prevent nail baiting. I guess this would also work on erasers.

Why do you want that? Is there some inobvious harm in chewing erasers? Or money is the main problem? Let him chew his own erasers then.

If the parents don’t support you, and you can eliminate the existence of mental issues that require treatment or special attention for chewie, and you can’t use a spray solution, I would go for gentle peer pressure. Point it out in class, do a friendly dressing down how none of the other students want to use the chewed on eraser. If he won’t stop if you say so, maybe you can get other kids to do the trick. The unwanted public attention from his peers might be enough. Would your principal be up for a bad cop routine where you can be the good cop?