You're not required to take classes during the summer. So even if you take 0 credits during a summer session you're still legal. However, you cannot be part time during a regular semester. There are some exceptions - medical reasons, last semester of your studies and I think there are a few others but you have to have documented reasons for it. You can ask your international student office - they have to fill out your paperwork so you have to talk to them - they should be able to give you more information....