Actors and singers do it by hiring a voice coach - someone who doesn't just know the sounds, but can explain how to adjust your mouth muscles to make them correctly.
Most classes and individual teachers won't do that. They'll either think "Eh, good enough for a foreigner" and shrug, or they'll say "That's wrong" and repeat the correct sound at you, which won't fix the problem.
Sometimes changes happen in one language. There is a huge difference between the Received Pronunciation (RP) version of British English that was the standard up to around the early 90s, and the Estuary English that became mainstream after that.
Most classes and individual teachers won't do that. They'll either think "Eh, good enough for a foreigner" and shrug, or they'll say "That's wrong" and repeat the correct sound at you, which won't fix the problem.
Sometimes changes happen in one language. There is a huge difference between the Received Pronunciation (RP) version of British English that was the standard up to around the early 90s, and the Estuary English that became mainstream after that.