@EasterIsComing
I am not experienced at bsja but a quick google throws up the starting course height for horses to be 90cm. This will generally be a fuller height course than an unaffiliated 90cm with more challenging lines and combinations.
I would want her comfortable competing at an unaffiliated 1m before she goes affiliated 90cm. The competition will also be more fierce, so only worth it if she is super competitive and ready for the challenge.
I don’t mean to be off putting just saying it isn’t for everyone.
If she does pony club or NSEA they offer similar leagues where she can build up points over a season or qualify for championships.
This is true, also what LastQueen says about the oxers, I'm too wussy and don't have the horses to BS these days myself but when I and teen children were members it def always used to be the case (and I'm sure it still is) that each BS course would have at least one oxer using the max allowance height and spread which would be pretty intimidating! Remember that the striding will also not be that 'kind', and may be set up for big horse strides so if her 15.2 is more of a 'pony' type in stride (as many good first horses for a child coming off ponies are) she may need to get used to that and accept he may sometimes struggle with the distances. And finally also the courses as well as being technical and up to height are usually very much 'dressed' with fancy painted fillers, plants, sponsor logos etc which can be off putting, and the venues will (in normal times) be busy, noisy places again with spook hazards like banners, spectator galleries and music playing over the PA - it can be a whole other experience to a low key local unaffiliated. We had the odd (very expensive!) outing to a BS end in total damp squib when horse took an immediate dislike to the arena and just wasn't having any of it which tried our patience (and bank balance) somewhat! Also do practice/psych yourself up to not being intimidated by people who seem 'fancier', on the SE circuit at least (maybe different elsewhere) you'll see a lot of people with all the bling, huge fancy lorries, massive strings of high powered horses (some of them clearly children of uber-rich parents, some more adult pros with sponsors etc). The thing to remember is in showjumping it really doesn't matter if you turn up looking scruffy with just the one horse of unknown breeding and a muddy tail, if you jump the fastest double clear you will beat all the shiny WBs and fancy jackets in the class, it's all down to performance at the end of the day... and there will be plenty of other amateurs/PC types just there for fun and the day out if you look for them, they just stand out less than the blingier ones!
I don't say all this to put you off, she should absolutely go for it, just so you can manage expectations and don't let her be disappointed if she has a few stops or poles her first time(s) out, I know plenty of teens put a lot of pressure on themselves and want to always win or at least place but BS is hard work and it's a good life lesson that success takes time and effort and doesn't tend to just fall in your lap (or so I always told mine anyway!)...