When I say (to non-Californians) that teacher's unions have too much power in California and am accused of being a right-wing wacko, I generally call this example out.
Unions have been responsible for much good (better working conditions, higher pay, etc...) but that doesn't give them a pass to screw students over.
It seems more Escalante was the one not prepared to compromise in this instance.
This is exactly the sort of thing unions should step in to solve, so you will have to find another example of why unions are not all good. There are plenty out there.
It's generally accepted that smaller class sizes help with educational outcomes (as well as teacher morale/quality of life), so it's not only understandable that there would be some agreed-on class size limit, it's arguably professional.
From the accounts I've read, the class size limit was pretty far from the most significant tension involved. Local Garfield administration and politics seem to have been. The principal under which Escalante's program flourished (Henry Gradillas) was a big contributor to the success, but even so, he wasn't invited back after a sabbatical, and it's reasonable to speculate that he'd run afoul of the community which employed him when he'd prioritized academics over athletics. Later principals weren't anywhere near as supportive.
There was also the professional jealousy that came with Escalante's hollywood fame. You can argue that's petty, but I hope anyone who'd do so from the software world is equally circumspect about any kind of professional jealousy when it comes to the success of approaches you don't subscribe to in the software world.
So, yeah. Unions as The Devil is one easy narrative, but it doesn't sit comfortably with the details of the story I'm familiar with.
That's even more heartbreaking to hear. Everyone's opinion seemed to have matter, except that of the students, whose careers and educations were at stake.