Well i guess you have two options. Once the plants are about a foot tall start the 12/12 light and watch to see which ones turn male. Kill the males ASAP and finishing flowering the females.
Option two would be if you want to grow larger plants and clear out the males. Once the plants are about a foot tall take a clone from each plant. Once rooted change the light to 12/12 and the little clones will show there sex. Now you can clear out the males and have room for the females to grow larger before you flower them.