Hi everyone i'm triying to add a button in a one2many view but i can't show it

my file.xml

  <group>
                               <field name="cost_ids"  nolabel="1" colspan="4" >
                                    <tree version="7.0" editable="bottom">
                                        <field name="mat" string="Matiere" />
                                      <field name="teacher_ids" string="Professeurs" domain="[('job_id.name','=','prof')]"/>
                                          <field name="coef_mat" string="Coefficient" />
                                          <field name="charg_hor" string="Charge Horaires" />

            <button name="return_action_to_open" type="object" context="{'xml_id':'action_school_unit_form'}" string="unites deval" help="Afficher le bulletin de letudiant" />

                                    </tree>
                               </field>
                            </group>