Can I apply for Tier 2 before workpermit expires??

[Alli2012]

I am a foreign national who started teaching on a permanent contract with a school in September 2006 on a working permit. My working permit was renewed on 31st Aug 2008 for 4 yrs ending on 31st Aug 2012. I am supposed to apply for ILR this year but unfortunately in February 2008 I was arrested for DD and given a £300 fine and 12 month driving ban.
As I understand this is seen as an unspent conviction which precludes me from applying for ILR.

Would it be possible for me to apply for a new visa under the Tier 2 General PBS before my old type workpermit runs out on 31 Aug 2012?
IF so, can I do this in country?
My salary is now £35,000 p/a. Also, my school has indicated that they are more than willing for me to stay. [/b]