"Watashi no KURASU ni wa onna no hito ga gonin imasu." is a correct sentence and can be also used as a description about your class, not just as an answer to a question "Nihongo no KURASU ni onna no hito ga nannin imasu ka?", so it's helpful in learning sentence structure (and it would be the aim of the textbook). "Gonin imasu" or "Gonin desu" is moe common as an answer, as I wrote, though.