In a school where the focus is on academics (I would put it different if I knew how better to explain our school. Our teachers are EXTREME in their focus on academics), classrooms stop using parents mostly by 4th grade. They still use them for field trips, costumes, projects that include spray painting or heavy duty glues, etc. Other than that, parents aren't invited to be in the classrooms. This isn't a PTO thing, this isn't even a school thing. It is a teacher thing. Pizza parties and the like are done in the lunch room except on rare occasions. Teachers here feel that 4th grade is where students learn to take responsibility for themselves in all things, from homework to classwork, to parties, etc. Sure, there are always some classes that still encourage parents to tutor, to read, to help with art, but for the most part they ask parents not to come to the classrooms.
Now, on the other hand, the lower grade (K-3) teachers are jonesing for help. They have parents who help out with parties and field trips galore, but few who are willing to come in 3 or 4 times a week to tutor. My son is in 4th grade. I tutor 4 mornings a week in a first grade class. I work with 6 students (in two groups of 3 now, but we started with individual tutoring) each morning. These are students who are way behind in reading. Once they are caught up, we switch them with other students who are behind. My husband helps with math tutoring in the afternoons 2 times a week. Our son knows that even though we are not helping in his class, that we are helping others who really need us. We help him with his homework at home, we have lunch with him and his class (in the lunchroom...lol...) a couple times a month. He knows I am on the school grounds most of the day volunteering in some way or another, whether it be filing stuff in the office, tutoring first graders, answering phones, counter help, running forgotten lunch boxes to students, helping the nurse with hearing/vision testing, etc. He takes pride in that and I think it helps him to do well in school. He sees me now and again and knows that I am there for every student, but mostly he knows I am there for him if he needs me.
Thing is, it is up to the teacher, not the parent, to have help in their classroom. Our district certainly encourages it, but leaves it up to the individual teacher. One of our Gifted Program teachers has never had a parent helper. He doesn't want one. EVER...lol... His whole focus is on academics and students. Hell, he even hates that he has to wait for morning announcements...lol... But his class is academically in the top 1% of the country.
I don't know what advice I'd give other than to maybe explain to parents that when push comes to shove, it is up to the teachers. Tell them to keep offering to help their childs class, but to find other ways to help in the school like tutoring a first grader who is behind in reading or math. Having lunch with your student and their classroom (in the lunchroom...lol...) is just as important to your child as helping in their classroom.
Good Luck!