You probably misunderstood what the teacher was saying. You are correct that moving the decimal point to the right increases the value and moving the decimal point to the left decreases the value. For example,
if I start with 12.34 and I move the decimal point to 1234, of course the number is larger. Perhaps your teacher was talking about units moving; for example,
if we write the metric system something like this
km
hm
dkm
meter
dm
cm
mm
If we change 10 meters to cm, we move the decimal point to the right and the UNIT decreases to a lower value (but the value of the number becomes larger). If we have 10 mm and we change it to cm, we move the decimal point to the left and the UNIT increases to cm (but the value of the number becomes smaller). If this doesn't clear things up, ask your teacher to please take a few minutes and explain that part of the metric system to you again. Or perhaps you could talk to your teacher during office hours.