Recording a class of children

For the song 'Geef Een Glimlach', a local school wanted me to do the recording, arranging, mixing and mastering. It was the first time I recorded a group of children in a recording studio. They were about 40. Since there aren't 40 headphones available in the studio (are there studios with that amount of headphones?), and since I did not have the time to record them one by one, I tried another approach. I put the group in the largest room of the studio, and put the part of the track on which they had to sing in a loop, through monitor speakers in the room. We rehearsed until the timing and tone were right, and then I switched off the monitors, let the group continue to sing while recording. After that, I took the best 6 singers and recorded them apart.