1) Don't go at all.
2) If you do go, stay outside the main hall, at least half of the time. Many do it. Some are tired, sleepy, others have a headache, small children, some clean the toilets or the outside area. If you can do (or pretend) any of these, you can stay out as long as you like.
P.S. The first assembly I missed, I had an awful (and long) pain in the stomach, and so I couldn't go. It was not a pretense, it was actually because of the assembly (having "woken up"). The problem was so severe (and continuing), that I had to take strong antibiotics. The thing is, we should watch over our health foremost, instead of pleasing others.