Yes, they are various versions of the first phrase, 'pulling your leg'.
2. No, I think you've misunderstood this part. At least, I read it this way:
Mrs Dodson has given him a score of 0/10 for his maths homework - which is incidentally on decimals. Since some of his answers are correct, and she's given him no marks, she doesn't know much about decimals. I think there is no connection between the score and decimals as such. But the wider context might suggest otherwise.