you can get work experience, it just isnt easy and you need to start early on it. prepare a CV and personalised covering letter and go round your nearest 10 or so practices. Its harder to reject you in person.
also consider NHS community dental and hospital dental departments
for volunteering - do the same, knock on doors, but perhaps see if you can get help at school to give you some support/referrences
school itself also usually want volunteers for open days and things like that