During warm ups everyone vocalizes the fullest range of the voice (both girls and boys). We do at least one overall ascending exercise, one descending exercise, and either a range extender or a tongue twister. I teach them why we do certain vocalizes and what their instrument is as scientifically as possible; that knowledge gives them responsibility and accountability for their participation and performance in class and on stage. For instance my favorite warm up is what I call a lip bubble (aka motorboat sound). Ascending and descending the perfect fifth, either with a legato or glissando articulation, gives the students a limited range to manage or focus on. This exercise is wonderful for supporting and maintaining airflow as well as relaxation of many muscles. Occasionally adding the outstretched tongue, which can release some minor tongue tension, injects some purposeful silliness at the beginning of the rehearsal. I usually begin in E flat or E and ascend by half steps to D’. If the piano is used at this point, I try to only have the open fifth or adding the playing the do, re, and sol as a chord to get their ear active in tuning.
As for voicing here’s how it works in my classroom. I teach them a short song or fragment and we sing it in multiple keys. I then bring the students up to the piano in small groups, eight to ten at a time, always of the same gender, and we sing thorough them again. I call this a Voice Check (like a doctor’s check-up). No one ever sings by themselves (which reduces anxiety) and I move around the circle "casually" listening to the individuals sing. I then ask them to identify which key felt best for them. The students know that I always take their opinion into account when deciding their voice part and that they don’t always get what they want. Their voice part is determined by how many singers there are in the ensemble, their ability to match pitch, overall tone quality, range, and level of experience. Students sing the part that fits their voice the best.