Everything's within the law as long as both of you 17-year-olds are receiving at least �3.30 per hour (which, obviously, you are):
http://www.direct.gov.uk/Employment/Employees/ Pay/PayArticles/fs/en?CONTENT_ID=10027201&chk= 00/jcP
As long as the minimum wage is paid (and there are no breaches of such things as sex discrimination or race discrimination legislation), an employer is free to pay what he/she likes. Each employee's contract is entirely separate. If your employer wants to pay you �3.30 per hour and the other 17-year-old �100 per hour, he/she is perfectly free to do so.
While there has been no breach of any legislation, it might simply be that your employer has made a mistake. Unfortunately, I suspect that the mistake might be that the employer has accidentally raised the wage of the other 17-year-old to the minimum rate for 18-year-olds. You might not be too popular with your colleague if you point this out to the boss!
Chris