My guess is that the fields are small enough that no effort is made to
compensate for them. A small bulb will draw ~0.1 A. The field of a long
straight wire is
2*10^-3 (Gauss-m/A)*I/r
so 1 cm. from a wire carrying 0.1 A you will find a field ~0.02 G.
The earth's field is ~0.5 G., so, in the worst possible orientation the
field will produce a deviation ~0.04 rad. =~ 2.5 deg.
If an LED is used for the light, the current is ~0.02 A., resulting in a
much smaller error.
It would be instructive to ask your students to perform the estimate
themselves