I think your physics teacher is mistaken. -100 is unquestionably less than -10, because $-100 < -10$. If you owe \$100 you have less money than someone who owes only \$10.
However, -100 is bigger than -10, because if you owe \$100 you owe a bigger amount than someone who owes only \$10.
Addendum: Some commenters have claimed that "-100 is unquestionably less than -10, because $-100 < -10$" is tautological. It is not. The first clause concerns the meaning of the English-language phrase "less than", which is what the original question was asking about. The second clause concerns the formal mathematical statement that $-100 < -10$. I am asserting that the phrase "less than" is normally understood to mean the mathematical relation denoted by "$<$". The conventional reading of "$<$" as "less than" or "is less than" supports this claim. In contrast, in the following paragraph I am claiming that the meaning of the English-language phrase "is bigger than" (and implicitly, "is smaller than") is not modeled by the mathematical $<$ relation. (More precisely, it is modeled by $<$ only for non-negative quantities.)
These claims might be false, or poorly supported by evidence, but they are not tautological. I hope this clears things up.