The adult high school close to Summerset? But progress is limited and why you need a high school diploma anyway?
But to improve English and fast, a complete immersion is required. But it is a very mentally difficult thing to achieve. Do you have the courage for the hard work? (like not have easy access to Chinese media or food for one year? Read, speak, think even dream in English)
If the cost is an issue, I suggest you to find local place to do volunteer work (e.g. food bank) where lots of English speaking ppl. Before you go there, try to recite the complete or big portion of ESL book such as "英语900句。" It is like "滚雪球”,you have to have some "materials" to work with in the beginning.