翻译:从中学起我已经是红十字会的一名成员了.

I have been a member of Red Cross since I was a middle school student.

I became a member of Red Cross when i was in middle school.

I have been a member of cross associate from middle school.

红十字会Red Cross Society
从中学起since my middle school.表达更简单
I have been a member of Red Cross Society since my middle school.