Once again, I had a version of this conversation.
Me: Charters are not public schools because they are not open to or committed to taking all students. They filter and cream and push out and refuse to backfill.
Reformster: What about magnet schools? Those are public schools which have stringent admission requirements. They don't educate every child, either, and they also pick and choose.
I can illustrate the hole in this argument by imagining the following contrasting conversations.
Student: I would like to attend your most excellent magnet school!
School: Unfortunately, you have not met our stringent entry requirements. However, as is our legal and ethical responsibility, we will make sure to provide you with a full education elsewhere within this school district.
Student: I would like to attend your delightful charter school!
Charter: Unfortunately, we don't want you. We don't have space for you. You are too old. You aren't a striver. You have educational needs we aren't prepared to meet. Or we accepted you, but now we've decided to push you out. Whatever the case, we are now done with you. Have a nice life, and good luck finding a place to go to school, because that's certainly not our problem.
Or perhaps these conversations.
Student: Is it true Mighty Swell Magnet School is closing?
School: I'm afraid it is, due to budgetary cuts and enrollment considerations, the district is shutting down MSMS and we won't open next fall. However, in keeping with our legal and ethical responsibilities, we have already placed you within one of the district's other schools. Don't worry. Your education will continue without interruption next year.
Student: Hey! This door is locked! How am I supposed to get to class today.
Charter: We closed. It just didn't make business sense to keep operating, so we are outies. Go away.
Student: But-- my education!!
Charter: Not our problem. Have a nice life.
In other words
Magnets are an extension of a district's system-wide commitment to educating every single child no matter what. Charters are not.