I am currently on an H1B work VISA. I plan to go back to school for a PhD and need to evaluate my options.

1) Is it possible for me to keep working full time without any change in H1B status and enroll in a school (that allows part-time PhD) and get my degree?

2) If the above is not possible, is this a situation where going to school part-time implies that I need to change my status from full-time H1B to another status (something that allows working and schooling for a non-immigrant alien resident)? If this is the case, does it mean that I will have to reduce my work hours from 40 to a lesser number (20 maybe)?

3) If neither of the above is possible, I am assuming I would have to quit working and enroll into school full time and apply for a change of status. Would this be correct?