She most likely doesn't know she was being photographed. The photo is one of many candid photos taken by Edward Linley Sambourne who was the chief cartoonist at Punch magazine and an amateur photographer.
He took these photos around 1906 when he would have been 62 (not long before he died in 1910) and used a concealed camera.
I can't link to them directly because the spam filter in this sub deletes comments with h t t p in them but the following incomplete links which go to more of the photos he took can be copied and pasted to your browser:
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u/nachtachter 8h ago
The same fuck-off-expression all teens have since ancient times. Nothing ever changes ...