It could be both. Prop 13 is definitely a huge problem, it cut school funding significantly since the 80s.

But also the focus on equality of outcome instead of equality of opportunity.

I read a good book a while back that pointed out how much more we spend on special ed, which is aimed at the bottom 5%, compared to what we spend on gifted education, which is the top 5%. It asked why we would spend so much on one and not the other, especially since the ROI is so much higher for the top 5%. (It obviously skipped the whole "making our society better and helping those in need" argument since it hurt their argument).

Special ed is expensive because it's things like 'this student needs a full time aid'. The only way to decrease it is to basically abandon those children.