the phrase you need is
"how are you going to safeguard my child"
They have to provide evidence that she will be safe. Given the age difference, it is not unreasonable to think that she is vulnerable to further bullying.
You need to stop trying to get him excluded, which will get you nowhere with school, and start getting them to implement their policies with relation to your dd.
So get a copy of their bullying policy
Get a copy of CEOP best practice for schools
Get a copy of anything they have written wrt cyberbullying.
Think carefully about what you need for your daughter to feel safe.
for example, she is scared to go to canteen in case he is there. The school can insist he is segregated at lunchtime for her safetly. (if it is her who is offered the safe place to eat lunch, that is victim blaming)
BUT this cannot continue for ever. So for a few months until everything settles down.
Also, consider requesting a letter of apology. Make it clear that there are a group of 4 boys involved.
Also bear in mind she is KS3 and he must be KS4, so they are not that likely to cross paths that much during the day.
I understand why you are so cross, you poor dd has been shamed in front of the whole school.
BUT you are laying all of the blame at his door, and it really isn't that simple. She is goign to have to accept that the result of her choice is that she is now in this situation. If the police were to press charges against both of them, would you want her to be permanently excluded?