I often see people talk as if Snape genuinely tried to get Harry expelled from Hogwarts, and maybe I'm wrong here, but I never saw it as such? Honestly, I doubt he even wanted to get Harry expelled even, as his only purpose in life now is to avenge Lilly's death, and Harry is the best bet for it. However, let's ignore this, even if he genuinely wanted, considering all he knew, I don't think it's reasonable that he genuinely believed that he could get Harry expelled for silly stuff. I always saw it as like when a bully threats to kill someone, but everyone knows they will not really go that far.