B2 change to F1 back to China

Dear Teacher 8,

I am B2 in US and want to change to F1 in summer. I plan to go back to china to get F1 by 7/30, my I94 expiration date. but school starts 8/29. Do i still have enough time to apply F1 in China? Is visa officer going to question me why i want to go back to US so soon? is it possible to get rejected because i just come back from US and now apply F1 to go back? if i apply for change of status within US, i heard it takes 6 months to process, if the process time exceeds the date of school start, and the new start date will be in 4 months, is it for sure the application will be rejected regardless it's immigrantion office's delay causing the problem?  

Thank you so much for your help!