Catastrophizing
Keep in mind that I was actually much more severely affected at that age than my son by characteristics of autism (I had no friends at all, my son is often described as "popular," although he had a huge dip at the beginning of the year due to a developmental jump...which he's come back from. I've got a pretty good idea of how stressful school is - I was that kid who played alone in the dust on the side of the playground for my whole school career.)
DS loves to socialize, and often begs for more socialization after school and on weekends, and now that the other kids have a better understanding of him, they are more than willing. However, his lag in social communication is a huge drain on his energy, plus he really struggles with a lot of the sensory/structure issues at school. It's stressful. However, there's a big difference between it being so stressful that he can't cope with it and falls apart completely, and it being stressful but he's able to bounce back as long as he has appropriate "scaffolding."