How many girls/women in the hobby?

[VintageRacer]

I'm interested to know approximately how many (percentage wise) females race on a reasonably regular basis. At the few events I've been to there have only been a small number, about 1 or 2 in 50+ racers. Is this typical, or are there clubs/regions/classes that have significantly higher proportion.

As a science teacher, I'm keen to get more youngsters interested in the hobby but feel it is a very male dominated one currently.