Have your female singers, one at a time, sing middle C. According to Vocalist.org, it is best to use a keyboard when trying to find a vocal range. Middle C is usually the third or fourth octave C on the keyboard, while on a full-size piano, it is the fifth C from the bottom.
Instruct them to sing each note down the scale. Each note that is sung clearly is part of their range. Continue until they cannot sing a note comfortably. Write down the last note sung and where it is located on the keyboard.
Instruct the female singers to start singing at middle C. Play each note up the scale until they cannot sing a note comfortably. Write down the last note sung and where it is located on the keyboard.
Count the octaves from lowest to the highest note sung, excluding the black keys. According to Vocalist.org, an octave is 7 notes. The octaves plus the extra notes sung equal that person's vocal range. For example if the lowest note a person sang is D below middle C, and the highest is G above middle C, their voice range is 1.5 octaves.
If the singer's range falls between G below middle C, or G3, and F6 to high C6, the C directly above middle C, then they can be classified as a soprano. If their range falls between C3, the C directly below middle C, and C6, then they are an alto.
Repeat the same exercise with the male singers. If their vocal range falls between C3 and C6 or above, then they are a tenor. If their range is G below middle C to F, G or B below high C, then they are a bass or baritone.