This may not be the answer you are looking for but it would certainly be a computer. Computers aside, it would be a mix between star charts, sky calendars, astronomical almanacs and telescopes (most of these items can be accessed using a computer).
The most useful tool would have to be diamond pickaxe.
A microscope is the best tool for a student to use to observe mitosis in an onion root tip.
vulnerability scanner
The finance tool on the most popular websites allows you to calculate amortization rates on several loans including automobile. You can also do home mortgage and student loans.
Most likely NOT, that would be glaziers tool.
It is a tool of measurement, a standard by which a student's academic performance can be measured. Without a tool of measurement, there would be no way to know exactly what knowledge a student has retained, and in some cases a student's ability to apply that knowledge by practical performance.
this will be a telescope because a microscope is to look at small things with and a telegraph is basically now a commericial
thermometer
Ping Sweep
Hand lens
There are several, but the most important ones would be saws and peaveys.