There is always more to be said, but I have a past answer that relates to your question:

Did Flappers use birth control?

To quote from the beginning - First we have to start with: what is a flapper?

The most likely origin [for the term] that I've come across is the one regarding young birds flapping their wings, mainly as I have come across "flapper" defined in a late nineteenth century English dialect dictionary as both "a young partridge just about to fly" and "a girl of the bread-and-butter age"; Barrère and Leland's cant dictionary of almost the same year notes that there's another meaning for "flapper" or "flipper", making the same metaphor but taking it in a dark direction: "very young girls trained to vice, generally for the amusement of elderly men". (So far, I'm a bit in the dark as to the origins of "bread and butter age", but from context in the late Victorian era it seems to refer to the early-mid teens, when children can be cranky and oversensitive a lot of the time.)

By 1910, it was in somewhat common use for young teenage girls. It turns up, for instance, in discussions of clothing to refer to girls old enough to have developed a figure but young enough that they can't be allowed to dress in women's garments yet - the Theatre Magazine put it, in August, 1915, "And it is no easy matter - this finding of suitable wearing apparel for a miss of tender years but developed growth. One is so apt to swing a trifle the wrong way." Today, many girls have the same problem: they don't fit into clothing in the Juniors section, and Misses can be too matronly or too revealing. Some took the term in a more judgmental direction (TP Weekly, 1914):

The Flapper.

He presents an extreme case - the spoiled girl [whose age runs from thirteen to seventeen] of moneyed parents, whose clothes must have a distinctive Parisian cut, whose manners are formed from being allowed to meet everybody and to air her opinions with the best, and whose tastes have been destroyed before they were formed by over-theatre-going and over-indulgence generally.

While the term was sometimes used generally for women of this type regardless of their age in the 1920s, well into the decade there are references that clearly play on the "too young for marriage but not a child anymore" categorization.

As I explained in the bulk of that answer as well as this one, writing about and studies of sexuality and birth control during the period focused specifically on married women, with the young women who engaged in casual sex tending to be of the urban working classes and generally considered "delinquents". What was considered a normal level of envelope-pushing for rural and middle/upper-class young single women roughly in the "flapper" category does appear to have been kissing for fun in the moment, spending time with the opposite sex unchaperoned, and sleeping with men they were already engaged to - certainly big steps from previous generations' norms, but rather tame by our standards.

As far as shorter skirts go - this is again something that's been somewhat overstated in order to be more relatable to late twentieth/early twenty-first century people. During World War I, women's skirts rose to at or just below mid-calf; in 1922, they dropped to the ankle, and they rose again in late 1924 (trickling down from high fashion to everyday dress by 1925) to below-the-knee. Some fashion plates appear to show shorter dresses due to the proportions of the drawn models, which sometimes have elongated shins, and some mannequins are made to modern fashion-model specs and don't allow the antique dresses to sit on them properly, but the general length for all women who kept up with fashion to some degree was just below the knee. As with the "loose morals" discussed in the above paragraph, this is even conservative by today's standards, but was shocking and new in the period - but still normalized.