In the 1980s I was a computer coordinator and data processing manager in a school system. (I was also a science, chemistry and math teacher for 10 years.)
When the comptroller put together our budget for the year he used hard numbers whenever possible and never used optimistic estimates of teacher absence, vacation days, or utility consumption. Using such unrealistic data to project a budget is looking for trouble. Our budget was presented to the Board of Education where heated discussions always resulted in repeated revisions. Exceeding the budget just did not happen.