Why was simon breathing funny when he pushed laura into bathroom?

Tomas doesn't have breathing problems though. Watch the end sequence where he walks into the dormitory- we see him walk quite normally and hear no such respiratory problems. The strange 'tics' he seems to suffer from at the party sequence are not apparent and neither are they apparent during the super8 footage.

When you wear burlap over you're face, it's probably really hard to breath.

I think we have to accept the fact that the line between the supernatural and real-life is blurred in the movie, intentionally by the filmmakers. Perhaps there isn't one answer to whether or it was Tomas or Simon in the mask at the time. Clearly, since Simon is found wearing that exact mask/costume, it is easiest to say it was him, but maybe there was also a supernatural element at the same time?

He was still crying (sniffling) and upset at his mother for slapping him. That is what that noise is from IMO.

He had a sack over his head.