Well lady's maids were quite a specialised position and unmarried girls wouldn't have had an official lady's maid of their own so if the Blishwick school did sponsor some girls for training it would be for general maid-ing. Lady's maids only answered to the lady of the house in a household so the daughters of a household wouldn't have one, however if a girl is trained above her station in life then she might not be considered suitable for housemaid duties in case she gets ideas.

So I'd still go for just having maids that were hired as maids to look after the girls' dressing and mending needs but either way I'm very into having more servants IC.