A teacher spends presumably most hours actually interacting with students and use the computer for preparation and paperwork something your 10 year old dell is probably well suited for during the minority of the time they spend on it.
Your dev spends most of their time on their machine and even if they don't have to build software in a compiled language may still be running a heavy development environment, several browsers to test the result, virtual machines, etc etc etc.
To drive the point home lets consider the cost of a 5% decrease in productivity due to using an inferior machine.
If a teacher is contracted to work 185 days or 37 work weeks in a year and earns 60k the teacher earns $32 at 50 hours.
If the teacher spends 10 hours per week on the computer the cost is no more than $32 * 37 * 10 * 0.05 = $592
If your software developer is earning 100k x 50 weeks and spends 50 hours per week almost all of which is spent using the computer then the cost is $40 per hour x 40 hours on the computer x 50 weeks x 0.05 = $4000
This doesn't account for actual costs incurred by having to hire more developers because management is too incompetent to retain them buy buying them nice tools.