School proms are relatively new in the UK.
Traditionally, British schools had things like:
A leavers' disco
A sixth-form ball
A school dinner or awards evening
The American-style "prom" only really became widespread in the late 1990s and 2000s, helped by US films and TV shows such as American Pie and High School Musical. Schools and parents started adopting the idea of a big end-of-school celebration with formal dresses, suits, limousines, professional photos, and fancy venues.
A lot of people have frustration with the cost. What was once a simple school dance can now involve:
£100-£300+ dresses
Suit hire
Hair and makeup
Transport (sometimes limos)
Tickets for the venue
Professional photography
For some families it can easily end up costing several hundred pounds for one evening.
The odd thing is that much of the pressure doesn't come from schools themselves. It's often social media and comparison with what other students are doing. A perfectly normal £30 dress can feel "not enough" when everyone's posting glamorous photos online.
My daughters age 41 and 33 didn't go to theirs.. l was happy with a leavers disco..
We seem to be becoming American.. with Baby showers too.