Usually having the "S" in the name implies that students are members. So, you need to check what your bylaws say about having students as members. If they are not currently allowed, then you would need to change that wording to give them membership privileges.

We are a high school who wants to expand our PTO to include students, making it a PTSO. We have just recently been established with a tax ID number and finished our bylaws. What is the process for changing to a PTSO so we can be more representative of our stakeholders?