There could be mismatched brightness if the
bulb resistances do not match. Because they are
in series the current is the same in each bulb,
but if one bulb has a higher resistance it gets
hotter since P=I*V=I2*R and hotter
means brighter. The teacher should get a
multimeter and measure bulb resistance. They
should also check if the same bulb is brighter no
matter if it is the 1st or 2nd bulb.
There could also be something weird going on
with their setup, if there are pictures I can take
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