iirc they are only first years, which is beside the point. While true their combat power is a factor, adults being adults would rather the kids stay in a safe place. Because the author needed a plausible excuse to include their OC, they wrote in a shortage of combat personnel and recruited the more capable of the third year students. Sachi and Myr get a pass because they have been involved since the beginning and have "shocked Pikachu face" convenient spells that can help pinpoint their location in the dungeon.