Is it good to give out your body like that? I don't know, it made me uncomfortable.－.－

in art schools its common for nude models to come in ^^ its just like a second job

Is it common in Japan?

I believe It's fairly common in most countries. It's a paid job, they don't do it for free. In regards to morals, i guess you could compare it to being naked on doctor's check up, while interns are observing.