The biggest class size my DCs ever had throughout their whole school careers was about 25 students. The average class size for them in high school was about 17.
It's hard for me to imagine, but back in the baby boom years their elementary school (RC, K-8) and the high school (public) had double the number of students they currently have, and classes were huge. Even now, there are three lunch sittings in the HS.
I can't speak for the rest of the US, but locally, SEN provision is excellent in the public schools. They have SLTs and OTs on staff, and both the elementary and high school districts have ed psychs on staff for diagnosis and creating IEPs. All schools provide classroom aides if needed, and children can get individual OT or SLT during the school day. The elementary and HS districts provide door to door school bus transport for all SEN students. SEN students can be mainstreamed or can do specific special ed classes. SEN students can participate in mainstream sports or special ed sports leagues. They can stay in school to age 21, doing life skills, academic courses, and work placement. Schools are obliged to provide an appropriate education for all students regardless of ability, and my local schools take their obligation seriously. It's a very MC-UMC area.
The local high school also provides a nursery for babies and preschool age children of students, staff, and the local community. There are academic counselors, licensed clinical social workers for students needing therapy, and deans of discipline who work with teachers, therapists, social workers, and counselors to help kids get through their school years.
The high school district contracts out the education of some kids who can't be mainstreamed because of social/ emotional/ psych issues (non SEN) to a couple of small alternative education providers, and provides transport door to door for these kids.
In addition, there is an in-school suspension department where kids who have committed serious rule infractions are kept apart from the rest of the school and do basic level core coursework, do therapy sessions, try to get back on track.
There are no metal detectors at the doors of my local schools. There is a community police officer who visits every day, and there are security guards who act as hall monitors and also do crossing guard/ traffic control duty on the streets around the school. They also monitor the bike racks.