Ukelele lesson

This is Josh, my ukelele teacher pictured here with another student in our beginners class. We've only been at it for 4 weeks, but already we have learned a handful of chords and strumming patterns, and can play a couple of songs passably. Josh is threatening to take us to the local pub on our last lesson before Christmas, to serenade the drinking folk with some festive tunes. I have reservations about this plan as I haven't yet mastered the art of playing and singing simultaneously. In fact it seems an almost impossible task. How does one sing when one's jaw is clenched tightly shut in concentration.