I've known many HS teachers who have joined and become valued members. I think it is most useful for those contemplating further graduate study and seeking contacts and research ideas and such. Some have gone on to earn doctorates.
However, for your stated goal, you will find that the members there, while not focused on secondary education, will have a lot to contribute to your programming and general CS classes.
SIGCSE folk are especially interested in APCS. The curriculum gets developed by many of these people, as do supporting materials (case studies), and many, many of them are AP graders.
CSTA, of course, is more directly focused on your needs. But SIGCSE can help you learn early of educational trends before they become embodied in curricula. You can even participate in the development.
For example, Greenfoot, an interesting Java environment for beginners, as well as BlueJ (more advanced), were developed by a team led by members (Michael Kölling and his team). He does tutorials, etc. at the SIGCSE and ITICSE conferences.