This happened late last year. To help pay my way through college I regularly pose nude for art classes. I'm a bit of an exhibitionist and at times get quite excited doing this. I'm glad I'm not a male because an erection is not as easy to hide as a wet, tingling pussy and erect nipples can be explained by the room being a bit cool.
One day Tim, the art instructor asked me if, for a bit more money, I would be prepared to do some slightly more erotic nude work for the camera class e.g. doing a strip while a being photographed. It didn't take much for me to say yes. Tim asked me to come dressed in a free-flowing dress with a bra and g-string. When I arrived, there were 10 students-6 males and 4 females. All had still-shot cameras except for a male and a female who had videos. Tim said he would play some sensual music and give me instructions. I was beinging to get wet already. First I just did some poses that included showing some skin and running my hands through my hair and over my body, including over my breasts and crotch. He then had me remove my dress and in just my bra and g-string continue sensually touching my body. He had me put my hand under my bra and massage my nipples then place one hand over my g-string and rub back and forth over my pussy then stroke with my middle finger the length of my pussy crack. He asked me if I was OK with what we were doing and I said yes. By this time I was quite hot and noticed that most of the men had bulges and the women had protruding nipples. He then asked if I would put my hand down my g-string and 'pretend' to masturbate. I didn't need a second invitation and I was not pretending. He then had me remove my bra and g-string and continue to 'pretend' to masturbate. I think it soon became evident to all that I was not pretending. I lay on the ground and just went for it. It was one of the most erotic experiences I have had, being the only person naked masturbating in front of clothed men and women. When I came, I did so with tremendous force and sqirted out some cum. It was an awesome orgasm!!! Afterwards a couple of the men and one of the women gave me their contact details. I think I will be have some very interesting engagements coming up.