It is absolutely certainly legal for what you describe to appear in literature. Consider, for instance, that Juliet was 14 when she married and had sex with Romeo. A more modern example: Song of Ice and Fire; Daenerys is 13 when she is married off to Khal Drogo, with their sex receiving multiple descriptions.
For a milder example, similar to what you actually want, look at this description from Dresden Files, of a character who is explicitly, only a couple of pages earlier, stated to be a juvenile, and with the first-person narrator being in his 30s:
Molly stood facing me in a long, gauzy black skirt, shredded artistically in several places. She wore fishnet tights beneath it, showing more leg and hip than any mother would prefer. The tights, too, were artfully torn in patches to display pale, smooth skin of thigh and calf. She had army-surplus combat boots on her feet, laced up with neon pink and blue laces. She wore a tight tank top, its fabric white, thin, and strained by the curves of her breasts, [..] Bright rings of gold gleamed in both nostrils, her lower lip, and her right eyebrow, and there was a bead of gold in that little dent just under her lower lip. There were miniature barbell-shaped bulges at the tips of her breasts, where the thin fabric emphasized rather than concealed them.
I didn’t want to know what else had been pierced. I know I didn’t, because I told myself that very sternly. I didn’t want to know, even if it was, hell, a little intriguing. (Jim Butcher, Proven Guilty, chapter 8)
In fact, there's no age rating for literature, like there is for movies or games. So it's not like your story could get "PG13-rated" for what you write.
Since real 15-year-old girls do dress in extremely sexualising ways, you are doing nothing but describing the truth. And 15-years-olds, who are perhaps your target audience, are already familiar with sexual attraction, even those who are not yet experienced with anything beyond attraction.
In light of all the above not only is what you want to write legal, I also don't see anything remotely wrong with it.