I didn't care for Celia's advice to Colin on how to deal with bullies. It's the job of teachers to protect students from bullying, not give them advice and then leave them on their own. He could still have gotten self-defense tutoring without that.
I like that Severus just can't leave alone the mystery surrounding the new teacher. Whether he realizes it or not, I think he misses spying, or he wouldn't be so eager to start doing it again.