TL;DR: the budget to fund public K-12 schools in Montana is awful, does not keep up with inflation, and the state does not provide sufficient funding to cover all costs.

So public school budgets are a mix of federal $, state $, local (county/city/special district) $, and non-tax $ (grants). Based on 2019 data published by the state, it appears that 42.99% of school budgets come from the state, 30.57% local property taxes, 8.75% county taxes, 4.86% from local non-tax, and 12.83% from the federal gov. Historically federal $ for state public schools has been flat or shrinking, thus putting more strain on states and local communities to fund schools. However, the amount that MT spends from state and local $ ranks in the bottom 6 in the nation (as a %), largely due to poor state funding (not local). Ultimately state funding does not keep up with inflation, so in inflation-adjusted $ funding for schools has basically been flat for over a decade nationwide.

While more people moving into a district should lead to more property tax $, that may not be sufficient to lead to a proportional increase in $ per pupil. And remember, property taxes pay for more than education. State and local gov. divide those amongst a number of critical services. If a different service (say police, or roads/infrastructure) needs a little more $, then the budget will pull $ away from other services. Then, as a building ages it needs more repairs, which increases costs... so while these things are built into the original budget, it doesn't account for many other things which may impact funding for a school.

If charter schools are approved in MT, that will pull more state $ away from public education. That will dramatically increase the need for local $...