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I suppose, if you want, you can assign all problems and write all exams
using SI units. I don't necessarily recommend that, but I don't really
care very much.
On the other hand, I think teachers make a big blunder if they extend
"use only SI units" into lab work. The reason is that common lab
equipment frequently does not read directly in SI units. Thus, if the
student is going to record SI units in the lab notebook she has to do
"on the fly" conversions between the instrument and the notebook. Doing
this is a very significant error in experimental procedure. This
violates the rule of "always write down exactly what the instrument
reads... do not make mental conversions."
Examples... Our air tracks have centimeter scales on them. Our
balances read grams. Our air-track timing systems read milliseconds.
Our thermometers read Celsius, and we do have some Fahrenheit
thermometers.
When students determine the mass of the glider and other associated
weights, they need to record grams, that is, what the balance reads. A
mental conversion to kilograms before recording the data frequently
leads to decimal-place errors.
When the students read photogate positions along the track, they need to
record centimeters. If they make a mental conversion to meters they
will make decimal-place errors.
When the students read the timer, they need to record milliseconds;
otherwise they will make decimal errors.
I shudder to think of the errors if their rulers read inches and they
are told to record meters, or their thermometers read Fahrenheit and
they are told to record Kelvin.
Major point... record what the instrument says, even if that is not SI.
If you need SI results, do the conversion later during the analysis
phase.