Last year my English class was full of characters. That's a polite way of saying it had some unusual people that I'll never forget. One student was such a hard worker that he learned all the idioms in our book by heart. He was always saying things like "I'm on cloud nine," or "I'm green with envy." We never knew if he meant what he said or if he was just practising English. Another student had a sweet tooth. She would bake lots of breads and cakes and bring students met in the class and fell head over heels in love. We were all invited to their wedding and had a great time. Then there was a student who was always pulling someone's leg. For example, one day before class, he put a long homework assignment on the board and made us thinkg that the teacher had given it. We all had long faces until the teacher came in. Then we realized that someone had played a joke on us.
by heart - by memory
to have a sweet tooth - to like sweet foods very much
head over heels in love - very much in love
pulling someone's leg - to joke, to kid or trick someone
long faces - a sad, dissatisfied expression