This is an awkward edge case. Try defining the teacher's answer using the inert function stackunits(num, units), e.g. stackunits(0, m/s^2). If you define a variable ta:0*m/s^2 this gets "simplified" to zero (quite reasonably) and your units get lost.
See https://docs.stack-assessment.org/en/Topics/Units/
Chris
See https://docs.stack-assessment.org/en/Topics/Units/
Chris