Spoofing is, by definition, not jamming but in terms of the goal if you combined a multi carrier spoofer with a DAS you could theoretically get things down to ~a million $ per high school on average and not horrifically leak signal as if you just stood up a spoofer and centrally blasted it out. It's also still carrier and technology generation specific though. I.e. if a new carrier/MVNO appears or an existing carrier rolls out new frequency bands then you need to replace the system. If it's a new technology (e.g. 5G) then you need to replace the system. If things just change in general (new MVNO on existing bands), you at least need to maintain the system accordingly (but might not have to replace it). You'd also need to convince the majority of people this is something all public companies should be mandated to support, which is a separate issue from convincing the majority of people this general approach should be taken, or you'll never get much compliance. All of that accounted for, the solution still brushes against needing to fix physics a bit in that lowers SNR in a many times larger area than the accepted dead zone - though not as badly as a jammer system.

Geofencing with cooperation of the local carriers is technically possible (with big downsides) in that if a phone is connected to 3+ towers you can decently get the position tower side to within several hundred meters (or worse) in an urban area. That means to reliably geofence a building you're still going to screw up anyone in the surrounding city blocks trying to drive by, use their phones at home/work, and so on regularly. It also still impacts noise (therefore connection quality), though not as bad, over a much large area than is excluded. You could also theoretically mandate that all devices have device side geofencing which uses the extra data from phone side (more than just tower connection data, BSSID or other local signal data) were you convince people to mandate public companies on this one as well (and even then there will be "old" devices for many years a la retiring 3G lest you want to convince everyone to throw their current devices away on top of the extra forced hardware monitoring ability). The cost for this would largely escape the schools by becoming a cost to carriers/hardware manufacturers.

Or you could, again, just build schools with RF shielding if this is really the path you want to go down. It'll probably have a higher up front cost but it'd achieve the goal for every high speed cellular service current and future without requiring any external mandates or external interference. It's still a bit silly compared to other funding requests schools typically have but it would solve the problem if the public prioritized it.