Translation:The younger one is my teacher.
A 的 phrase can stand in for a noun in Chinese. Or rather, the noun for said phrase can be omitted (in this case: 人/老师), especially if it appears twice in the sentence, similar to how we use "the one(s)" in English.
In English "one" is optional in comparative sentences like this. But if it's included it can never be written as the numeral "1" as Duolingo suggested to me.
比较 means compared to average. It usually gets translated as an -er suffix, or the adverb "relatively".
this sentence is so confusing to translate bc the english comes out as a fragment oof