They could have just let Natsuki taste it. It's not like such a dutiful student would instantly turn into a delinquent from one taste of booze. The forbidden fruit is always more enticing anyway, so it's best not to let kids think it's something marvelous they need to dream of. That's why wise parents let their kids taste some especially astringent wine or bitter beer, to make them think alcohol isn't some nectar of gods kids are unfairly deprived of.