It hurts my really bad to sit in a hard chair for more than an hour?

and for drama play rehearsals right now we have to sit in a circle reading the script for 3 hours at a time (its a loooooooooong script) what can I do? I was about crying by the time we got to get up but i don't want to draw attention to it because I know it hurts me because my s too bony and im naturally really skinny and a lot of people already think I have an eating disorder (i dont). Like literally these 2 girls had to do a skit in my cl and they chose this one about bulimia and like half the cl whipped around and stared at me and my friend Robin because I'm the skinniest girl in the whole school and she's the second. So what can I do? sitting on my coat doesn't work ( i tried it) because even though the coats really fluffy the chairs still hard underneath