I would usually agree with this.Sounds pretty cruel to me. You could keep them in the same conditions and monitor their growth with certain foods instead. For example, feed one some foods, feed the other different kinds. Compare growth, behavior, color.
I do see an opportunity to possibly educate her fellow classmates, teachers, etc. about the true needs of goldfish though. It may sound ghastly... but maybe keeping one in a small bowl short-term and being able to document the impact of proper conditions could have some impact down the line. Be sure to mention or illustrate how large a full grown "feeder", "carnival" goldfish should get.