Weavers are one of the most effective workers possessing skills and are hard working.

Correct sentence- Weavers are one of the most effective workers, possessing skills and are hard-working.

The subordinate clause "possessing skills and are hard-working" must be separated from the independent clause by a comma.

Until the principal dismissed a student from the class there were twenty students.

Correct sentence- Until the principal dismissed a student from the class, there were twenty students.

The introductory clause must be followed by a comma.

When the hot, summer season sets in, many people believe that they suffer from sunburn that is related to changes in general weather patterns.

When the hot summer season sets in, many people believe that they suffer from sunburn that is related to changes in general weather patterns.

Multiple adjectives describing the same noun should be separated by a comma. In this case, "hot" and "summer" both describe "season". No comma is necessary after the last adjective.