Traditionally male, more common on girls than boys these days, but I would not assume female if I saw Riley on a class roster (as I would with other traditionally male names that became more popular for girls like Lindsey and Dana)
I know quite a few Rileys, all girls. It's a name that tends to be listed as "gender neutral" on baby name lists, but like Lindsay, Courtney, and Kelly, (all supposedly gender neutral), Riley says girl to me.
Re: Riley