If the small shop happens to be collecting sensitive personal data -- health data, political affiliations or union membership, etc. -- (a) they should probably stop and (b) they would need a DPO.
Highly unlikely in any case !
As to the DPO, it is meant to be an independent person within the organisation, who cannot be unduly influenced by management, etc. Assigning the role to a regular employee who, e.g., reports to the principal and is a teacher by day, would not seem to satisfy the requirement.
Bear in mind that a DPO has to have "expert knowledge of data protection law and practices and the ability to fulfil the tasks referred to in Article 39".