In order for teachers / pupils to join by responding to the Join Class Link, check Allow joining by link under Students > Add Pupils, or in the case of teachers, under Teachers > Add Teachers.

Once this is checked, anyone visiting the link can join the corresponding class and / or school.

Joining by link is convenient and simple enough to be used by children, however the flip side is that anyone using the link can join your class / school. Consequently, as soon as everybody who needs to join has joined, Allow joining by link should be unchecked.