I took over/became the President of an EC-12 PTO 4 years ago. It ican be extremely difficult to get response from the higher grades, but does get a little easier or your expectations simply decrease. For a couple years we asked for a representative teacher to attend meetings and that worked out well - till the year ended.... But a least we started the seeds of communication going!
Some things we have done that worked - teachers need to be present when they make a wish request (wishes are for items teachers need for classroom/instruction) - this gets them into our meetings and we can meet them face to face and ask questions about why this is important to them and why they came to us.

We have also started "hiring volunteers" from the high school/middle school. Since we are not planning events for them, b/c of all they are involved in already - we ask the organizations they are in if they would like to work at our events like Fun Fair and other activities. In exchange for their work we pay their group a modest pre-determined amount. Now you may think wait your are trying to raise money, but in the end it helps everyone out - there might be one less cookie sale, or wrapping paper sale, the PTO looks great b/c we are supporting the high school and middle school organizations, and it allows the elementary parents to enjoy the event with their children instead of volunteering, win-win!
Also have held cheer clinics for the Elementary school students with the guidance of the middle schoolers, then held a huge school pride night at a football game - brings the whole school and many different organizations, and turned out great - also did this for basketball. Gets the younger students seeing what to expect as they get older, as they will be the next generation of the athletics.

We always include the higher level teachers in on everything - we still ask for Teacher Experiences for the Dinner & Auction from them, (and many respond), send them our newseltter via email to keep them in the loop, and make friends with the secretaries in those schools, b/c they can be some of your greatest help.

We have also had new principals start - so we have scheduled meetings with them to tell them why our organization is important and what it provides to the school.

The only time we have parents of older kids attend are those that have younger siblings as well - frustrating, but ......

Just know that you are making a difference!

Sorry for the rambles, hopefully some of this helps

this year we have gone from a 6-12 to a k-12 school. i am surprised at how much the 6-12 involvement seem sot have dropped off already. maybe it just feels like there is less because the k-5 parents are so eager. tomorrow is our general meeting and i'll see who shows up. thankfully because we were 6-12 before the principals are used to ptso and most of the teachers as well. so we aren't having difficulty connecting to staff.

i am interested in hearing more about 6-12 groups.

We have a K-12 PTO as well, although we just got started mid-year last year. We have about the same number of students and set up committees for each campus - 1 for elementary, 1 for middle school, and 1 for high school (HS.) The elementary committee parents are always the most active, middle school somewhat active and high school hardly any activity. After talking with some high school teachers, we realized that they just don't have time - with all of the clubs, sports and other extracurricular activities that high school students (which means HS parents and teachers, too), they are already swamped.
We still try to include them, but unless it's a program aimed directly at HS, we don't expect much response. We do try to provide volunteers (mainly elementary parents who want to help at any level) to help with things they're already doing in hopes of continuing to reach out to them.
One of the previous comments was spot-on as far as more moms working outside the home as kids get older - at least that's what we've found in our school.
We had a lot of support from our previous high school principal, but he has moved to another school, and we honestly haven't reached out to the new principal yet as far as a face-to-face meeting, but that's definitely on our to-do list.
I'm not sure how helpful my comments will be to you, but just know you are not alone.