All PTAs are open to membership including teachers and would in fact give them voting rights as a member.
However, most that I talk to don't have a large number of staff involved in their meetings.

Us too. We are a HSA and all our teachers are paid members. (Their association pays the dues for all of them!) They are eligable to vote.

Amend your bylaws if need be.

You should be able to ammend your bylaws to give the teachers voting rights.

Our teachers all join our PTO, so yes they do all have the right to vote.

We are a Home and School Association. It is my intention to give voting rights but our bylaws right now are not clear.

Are you a PTO, PTA, or PO/PA? We are private, episcopal day school and our by laws do not mention teacher voting either. However, we are PTO specifically because we involve and support our teachers. We do give them a vote on distribution of funds, projects, etc. I have learned recently though that most private schools have parent organizations only and do not include teachers. We are a small school and feel it is important that they have a voice and one of our primary goals is to support them.

I'm a newbie and our bylaws (the few inadequate ones we have) state that teachers are "supportive members" What does this mean? And do most groups give teachers voting rights?
We are private, catholic.