回複：8 teacher,still have enough time to get F1?

8 teacher,I got B1/B2 approved, may arrive in the U.S in middle of Dec 2011.

so if I want to transfer to F1, do I apply fall of 2012 or spring of 2011?

-Depends on how long your B1/B2 I-94 duration will be. The principle is that your status can not have a gap between your B2 status and your F1 status.

say if I apply for Fall of 2012, and I submit B1 TR to F1 in Jan 2012, if by April, F1 is approved, does this F1 allow me legally stay in the US before Sept 2012 school start? 

-No.

or I need to go back if F1 is approved, and come back in Sept 2012?

-Correct. But if you will go outside US to to get the F1 visa, you do not file the I-539 to change your B1 or B2 status within US and just get the I-20 from the school and go outside US to get the F1 visa to come back.

If I apply for fall of 2012, when shall I submit TR application to USCIS?

-Hard to say because we do not know exactly how long your change of status will be approved by USCIS. But let's say 3 months for I-539 approval and your Fall Semester begins on Aug. 20 (effective date of the I-20), then you may file the I-539 change of your status from B1 to F1 in the early of May 2012 (before your B1 or B2 expires). Then, you can stay in US until the Fall semester begin. Your F1 can be approved in advance up to 30 days before your I-20 is effective (Fall semester begin) and thus, you do not need to go outside ot get the F1 visa even if your B1/B2 I-94 expires before that time. 

for above case, if it is denied in April, my 6 (Dec2011 to May2012) months B1/B2 is still effective, and this denial won't affect my future b1/b2 entry, right?

-Correct if your B1/B2 I-94 is still valid when the F1 application is denied.

also, and all these b1/B2 TR to F1 won't affect my previous H1B, right?

-irrelevant each other.

I stopped H1B clock at the end of Oct2011.

Say if I apply for Spring of 2012, and I submit B1 TR to F1 in Dec 2011 or Jan 2012, if this F1 is approved in march, my spring school already started in Jan. Then how does this F1 work?

-Normally USCIS may not approve your F1 in this case.

if I have this F1, can I start school in Sept 2012,

then the time before Sept 2012 is legal stay, right?

-No. If USCIS approve your F1 applicaiton and the I-20 is effective and you do not register the full-time load of courses, your F1 is void.

Thanks very much for your advice in advance!