Not a teacher.
#1 is how I interpret it.
The "but" is contrasting two things about the subject "it" (meaning the new teacher's lessons).
It should read "It wasn't as fun as the old..." if you meant that the old teacher's lessons were a little bit fun, but not as much as the new one's.
Or "It wasn't fun like the old..." if you meant the old teacher wasn't fun at all.
- For Teachers