which is harder teaching high grades or low grades, why?

The answer is simple ,teaching low grades is much important bcz in this level u will have to build the base , and bcz u are going to teach them something new ,they dont know ,or they haven't study it before, so u have to work hard to make them ready for the next level in their lives .....

Yes teaching low grade students is more important , but teaching high grade students is much harder beacuse of the hard effort that u have to make to give them the information they need and your control over them is not the same as in low grades...

An efficient teacher will not find it difficult to handle high grades or low grades, since this skill is an inborn talent which a person has to possess, when imparting knowledge to fresh minds, which is imaginative, creative and are eager to gather new information.  

Therefore, a teacher should have effective communication skills and be creative enough to convey the message clearly to students, as all the children has different wavelengths of grasping new concepts.

Since it is a teacher's responsibility to mould the future generation's characters in the right way, it is very important that people with the rights skills are appointed as teachers.

Everyone can be good learners, through listening.  But only a talented person with the right skills can be an effective teacher, as teaching is more difficult than learning.