I'm 15 and my school dropped metal working starting this year.
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Check out your local community college. Don't know what the admissions restrictions or requirements are, but they have trade programs and I know the one near me has a pretty nice program in welding and other shop arts. You might be able to take evening classes or get classes during the day and take those rather than be in your high school. If advanced learners can take college level courses in high school, I think something could be worked out for this case as well. Talk to you school and work it out.
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if you from the uk you can do an apprenticeship at the age of 16 and also do your gcses in a part time course all for free and its nothing like school just an hour a week on a computer