Hi Chadaporn:
Your J-1 visa is valid until August 31, 2015. You are legally allowed to work until August 31. You can remain in the U.S. up to 30 days after this date but for traveling only. However, you must return to Thailand so you are there for the first day of classes at your university.
If you decide to come back to the U.S. to study, it should be fine as long as you get the correct visa to do so.
Enjoy the remainder of your summer in the U.S.!
Regards,
Valerie