Odd One Out:
Ruler, Pencil, Ball, Envelope

Explanation:

Ruler:
A ruler is a tool used for measuring and drawing straight lines. It is commonly found in classrooms and offices.

Pencil:
A pencil is a writing instrument that is commonly used for writing, drawing, or sketching. It is also a common tool found in classrooms and offices.

Ball:
A ball is a spherical object used in sports and games. It is not typically found in classrooms or offices unless used as a stress-relief toy.

Envelope:
An envelope is used for sending letters or documents through the mail. It is commonly found in offices and homes for mailing purposes.

Conclusion:
The odd one out in this group is Ball because it is the only item that is not commonly found in classrooms or offices for writing or mailing purposes.