hello , sorry been offline for a few days due to pc crashing . Excellent news, good luck to her. dd's class had several visitors last year too. They do a mixture of in-class activities and assessment in English and Maths by a teacher one-to-one. Nothing too stressful !
Just saw your query about 11 and 13. ds's year has relatively few girls (about 35:65, most are nearer 50:50) and many of them are planning to stay on at present. Much is made about readiness to leave at 11 and obviously you really ought to if you intend to go state for secondary but most independents have entry points at either. The year groups are smaller and friendly for Years 7 and 8 with lots of opportunities which might overwhelm them among a larger secondary intake at 11. Obviously it is in a prep schools interest to keep them longer , so they do a good sell on it and recruit into Year 7, however I think it is nice to have that option. Depending on where ds goes and how dd is doing we may yet decide to move her at 11 but 13 seems right for ds.