Iam a US resident wiht a job offer in Canada.
What will be faster way to process the permit?
If i get the work permit, iam allowed to go back and forth to the US during the work permit duration?
Can, the work permit be renewed or extended?

Thanks a lot in advance

Some of your questions are addressed in the Canada Work Visa FAQ.

Work permit holders are always permitted to exit Canada. The ability to re-enter depends on the possession of a valid entry visa or on being passport visa exempt (i.e., eligible to appear at the port of entry seeking admission without a passport visa) on the basis of nationality and/or permanent residence (a US citizen or permanent resident is passport visa exempt).

David,

does a school teacher require an LMO from their respective employer before they obtain a work permit

Thanks