It's really hard. DD had a seasonal hospitality job for 5 months as soon as she turned 16 (total fluke, right place at right time application). It then took a whole year to find her next job. She applied to probably 70 (well-targeted) jobs in that time - I helped her find the jobs, tailor the applications, complete the screening/aptitude/personality questionnaires, and prepare for interviews. Over that year she had 6 interviews - pizza express, garden centre, pet shop, country store, Argos, Sainsbury's. The first four the on that list she was pipped to the post by better teen candidates, the last two it was clear from the interviews they wanted over 18s with full flexibility to cover shifts.
She eventually got a job at Greggs. Which has been excellent as she's worked 6 hours a week through Year 13 and now her A-levels are finished she is working 2-3 additional days a week, plus has already got a student transfer arranged to her university city.
From this experience, I would focus your efforts on the independent businesses, where the hiring manager's own bottom line benefits from paying a teen less than the adult minimum wage, or chains that are known to take under 18s routinely - TK Maxx, JD Sports, Greggs, Pizza Express etc. Waiting jobs in chain pubs, in particular, were a total waste of time, never offered an interview, I think because they ideally want staff who can also serve alcohol.