I'm sorry to hear all of this, it's a real nightmare I am sure! Seemingly the issue is word against word as far as the school and Police are concerned and both seeing their hands as therefore tied.
Should it get to the stage where you meet the criteria to escalate it to the NI Public Services Ombudsman (link outlining below) open a case then direct MLA or even better supporting MP to chase them, nothing moves faster in the public sector than when an elected official is on their case. Likewise with the Police Ombudsman if you feel the action or lack there of would be incorrect on their part to date.
If not already done so, I'd strongly suggest via GP seeking a form of mental health assessment to establish the mental health impact side, this would help in demonstrating injury caused in all aspects, if this was then to worsen sue everyone involved.
With regard to body camera, have the Police suggested this in writing at all and specifically included the intention of within a school? Have the school been advised or consented etc, I'd hate to think that was the advice off the cuff by an ill thinking junior officer which very quickly could find other parents going mad and heading to media etc the headlines sensationalised. Echoing other comment on this, I would be fearful of this making her more of a target than anything else.
Is home to school transport provided under an Education Authority blue school bus pass? If so definitely chase them independently and not as part of a larger overall thing, make them start their own process too. All modern type yellow EA buses have internal CCTV now as do all TL vehicles. For either go directly to the depot concerned and require the manager to meet over it and always a plus to allude to 'legal advice' you may or may not have sought and been advised to approach them in this way (make them sit up). TL for example publicly big themselves up that all their depot managers are qualified child welfare protection officers... Put them on notice and move to doing all in writing ASAP to hold all to account. At the very least in either case drivers can in such circumstances be made aware and will keep a look out and in the event of something happening verbal or physical can at the very least be very credible witnesses where peers may not do so or CCTV of a sea of children proves fruitless. Behavioural issues can result in individual children being refused transport. Equally if under the EA home to school transport can be provided by taxi in certain circumstances, but not a solution in itself. The duty of care within that side is more or less independent from the school itself and will have its own policies and procedures. If paying for home to school transport by bus and it's a published timetabled school service, the same protections / duty of care would apply as having a bus pass.
Don't let it all melt into one big complaint where everyone is involved and no one thinks it's their problem as the other is in on it now. Schools policy, Police investigation, chase both and ask for an update and outline with timeline of what they have done to date (again "I have been advised to request......." , I'd keep any such interaction in writing via email only and to a specific persons work email address only as even a failure to respond would open up escalatory Ombudsman intervention (as little as that would achieve in itself it helps hold people to account). 'Lets sit down for a meeting' or telephone meetings etc leave no real accountability, if you feel it's not working stick to in writing only and make it clear why. Also if you have the support of an MP and or MLA keep in contact with them on as personal a level as you can, ask them if you can cc them into any email chains if for no other reason than for the other party to see they are acutely involved at all times and keep them focused. It's all too easy to think 15 yo only another 8 months of term time left until likely not our problem anymore...
Random one also, don't underestimate the power of an overweight over paid big mouth when on your side... Start pestering Nolan if you dare, call, email and social media begin any opening contact with something catchy like in the above 'bullies shot my....'. Genuinely that marmite of a man could actually make things happen in the space of a few days that following the lead of individuals yourself can be fobbed off indefinitely.
With regard to the 'BB gun' in Northern Ireland air powered articles such as this for several years now are legally classed as firearms and a firearms certificate is required (over 1 joule I think) broadly speaking anything merely resembling a pistol is banned now even paintball guns unless long barrelled additionally it is illegal to carry such a thing in a public place without lawful authority, even one below the low powered threshold would likely be classed as an imitation firearm. That would be something worth quizzing the officer dealing with the 'investigation' as it adds an entirely new element to the now banded around willy nilly 'hate crime'. Imitation firearm in a school setting might bring a different aspect of importance to how it's prioritised... Have they sought to locate the reported article at all and establish it's legality? Consult MP / MLA / Nolan on that one too.
https://www.nidirect.gov.uk/articles/dealing-bullying-and-getting-support