The school district wasn't blindsided. The county was.
I work at a school district, so let me tell you how public schools get their Internet. Obviously it's going to vary, but from what I've seen our situation is the most common.
Every building in a district runs fiber between their buildings to a central building where the district's servers are kept. It could be in the basement of the high school or some administrative building, but that's what happens. It's easy enough for the district to do capacity planning. Each school district may then connect to an intermediate school district, but ultimately then connects to the county's municipal network. This network is the ISP that connects every municipal office, such as city, village, police, fire, public works, etc. to the Internet. In my state, Michigan, the county connects to a statewide municipal network originally put in place to connect the public universities to the proto-Internet for research (our ISP, Merit, was founded in 1966).
In this story, the problem is at the county level, the middle man between the school and the state-wide network. This is not particularly surprising, since since in every case I've seen, the county is a) poorly funded, b) poorly staffed, and c) tends to be forgotten about. When a neighboring district went 1-to-1 at the high school level with lots of online classes, they did reasonable capacity planning for the district's small network and quadrupled the bandwidth from the high school to the district servers, and the district to the county (the district consists of 3 buildings on the same plot of land, so it was fairly simple). What they didn't do was consider that the county level needed upgrades as well. My district is about 8500 students across more than a dozen buildings, and this was about 200 students in just one building (grades 9 and 10). They were using about 90% of the bandwidth on our connection. That district got moved to a different connection pretty quickly, but until then nobody in our district could use the Internet during the school day.