Which class of students are hard to teach ? 1. nursery 2. high school 3. college

Nursery is a joy, high school is a challenge and college is about falling down and standing up again and again!

Nursery   :-)

Nursery

High School!! To a degree they are in "no man's land" and are also at that rebellious age! To Nursery children learning is just an extension of "play", College students should have an idea of what they want to do in life and should be motivated and working towards their goals. High School kids on the other hand are past the "it's fun to learn" stage and haven't quite matured into the "I need to get working as my career could be on the line" stage!

In nursery, they are curious, eager to learn providing the right motivation.

In college, they know learning is usefull, are self motivated.

But in high school, they tend to think they know best, they tend to prove their independence toward any kind of authority and that might be a problem. So I say high school is the most difficult.

It is something related to your aptitude and skills.

For some Nursery is fascinating but others prefer high school. Yet others will prefer college students to test their teaching skills. 

College.

Because the instructor doesn't know how bad is the guys may going to be if he annoyed them.

I remember a lot of things had happened before.  Too hard!!

I think it depend on the teacher her/him self. It you enjoy being with children and can tolerate their need to be active most of the time and to"step down" to their level, then nursery is easy for you. If you like to wander in the teens imaginative and "unrulled" world and ready to convince them to accept you as a friend in their world then high school teaching is easy for you. If you just like the matures world and love to be logical all the time, then go for college teaching. This is my own opinion.