Real spending per pupil K-12 has gone up but NOT for classroom instruction of mainstream learners. You’ve got kids who are in regular elementary school classrooms who weren’t getting an education at all due to their learning needs, physical disabilities, inability to attend a regular school program.
You can’t look at “per pupil” without looking at the high costs of mainstreaming, providing special services, vans and para’s, one-on-one aides, OT/Speech as “pull outs” during the day and not done at the parent’s expense privately, etc. My own school district spends more on lawsuits over accomodations than on many aspects of the actual curriculum. Overall spending on instruction is flat even though the budgets are going through the roof. Plus paying the costs for kids who live in district but are going to private school when their needs cannot be met locally.