OP, I'm a DSL in a school. I'm sorry this happened to your DD. Some of the advice on this thread is not good, but you are absolutely right to have expected more of a response from the HOY. The following is what I would expect to happen as as good practice.
I would expect the HOY to get a written statement from your DD, as well as any witnesses who could verify that the comments were made as the boys will likely deny it. I would then expect the HOY to find out if your DD is in any of the same classes as these boys and if so, ensure seating plans are adjusted so she does not have to sit anywhere near them. PP have suggested the boys should not be in any of the same classes going forward, this is an understandable assumption, but would not be logistically possible in many schools. There may simply be nowhere else to put them as you can only have so many students in each class.
I would expect your DD to be helped to identify a trusted adult/adults in school she likes and feels comfortable talking to so she has the option of going to them if she wants to talk about what happened.
I would then expect the HOY to gather evidence (review the aforementioned statements, speak to any witnesses), and speak to the boys involved and their parents about the allegations. A sanction (usually time in isolation) would be appropriate (and I would argue essential) if they admit to the comments, or there are witnesses who corroborate what your DD is saying. If not it's difficult because sanctioning a student for an offence without evidence would most likely put the school in violation of its own behaviour policy and of course most parents in this situation would believe their child and would not support a sanction without evidence.
Either way I would expect you as a parent to be advised that you have every right to make a report to the police if you wish to do so. Just to warn you though, IME the Police will often say they are happy for the school to deal with the matter in cases like this. But I have known instances where they've offered for a PCSO to come into school and talk to boys about their words/actions and what the consequences could be if they don't address their behaviour.
I'd expect the boy moved from every class of hers and to have to undertake counselling at the school as a requirement to attend.
Schools cannot force a student to undertake counselling, for any reason. They have to give consent. Counselling cannot be used as a punishment or an incentive. That's not how counselling works and no counsellor would agree to this as it would be a clear violation of their professional code of ethics.
I hope your DD is OK, OP. Peer on Peer sexual harassment is a massive problem in schools and schools have to be proactive in tackling it by including it in their PSHE curriculum, assemblies and tutor activities as well as just dealing with incidents as they happen.