First, there has to be an admission on the part of the teacher and anyone else involved in maintaining this tank that the fish species combination is not suitable from the perspective of the requirements of each species as well as the tank size. Once that is understood, some of the fish must be rehomed to other tanks, a store, a hobbyist. The aim being to only have compatible species suited to the tank space, the water temperature, the water flow, the aquascaping.
Until the above occurs, one or more of the species in this tank are going to be at a serious disadvantage, and that means poor health, ongoing problems, and to be honest inhumane conditions.