Awesome, if it hasn't been done already, which is hard to believe - although I can't find anything when it comes to music teachers. However, if it has already been done, but you can think of a better way of doing things, then perhaps there's still potential.

The idea is great but you'll have to figure out the following items:

How to verify the identity of music teachers and their respective grades for musical instruments?
How to verify the authenticity of reviews?
How do you rate somebody's skills when it comes to musical instruments because music is hard to judge for an average Joe?

Despite the questions above, I still think it's a good idea but you'll need to figure out ways to solve the questions above since they might be the first few questions that your site visitors have in mind.
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This is obviously nice idea, but first figure out many other such service providers apart from this drum/guitars/dance/art/ teachers etc. then you can provide visitors to put their reviews about them.
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