A group of tissues that work together to perform a specific function is called

An Organ

Explanation:

Under the biological levels of organization in living things (otherwise called cellular hierarchy) describes the order of cell organization.

The smallest would usually be a cell. (However some charts start with organelle first). Then, a group of cells would be called tissue. A group of tissues who share a specialized function / specific function is called an organ.

Next, a group of organs working toward a specific or closely related goal is called an organ system. Lastly, a group of organ systems work together to form and maintain an organism.