yekke2 – all unmarried high-school and sem girls, and all bochurim and I think also high-school boys, whether local or foreign, are bound by this takanah.
And never mind the story you heard firsthand – no offense intended – but I myself went into Stenhouse about five weeks ago, towards the end of winter zman, and there were around ten girls – yes, all American – in there, chatting and giggling noisily, and only after complaining to the woman at the desk did she feebly request that they vacate the premises at the earliest opportunity. Ten minutes later, they were all still in the shop, and at least three others had entered during this time.(!)
There is, of course, as well as simply trangressing the Rov’s takanah, the immense problem of causing bochurim to have improper hirhurim, which is the intended purpose of most of the latest gentile-designed fashion outfits that many American girls wear. The native Gateshead girls and those of the wider English community are generally more tzenius in their mode of dress. Of course these are generalisations and will not necessarily apply to all girls in any specific category, but they are the general trends that I have noticed of late.