This has nothing to do with violin making.

Ofsted inspect all Further Education provision and some specific lower-level vocational Higher Education provision (for example, HNCs in some skills-based disciplines).

If you are found to be unsatisfactory in your provision by Ofsted, then you are prevented from taking Tier 4 visa candidates until reinspected as satisfactory. It doesn't matter what the actual course is, them's the rules.

Originally a Home Office stick to beat fake institutions offering English through work experience, it's applied universally.