"My classmate is very tall."
I'll repost my comment here: This sentence has a Topic-Comment structure.
Subject (我的同学) + Topic (个子) + Comment (很高)
我的同学很高 is a perfectly acceptable sentence. However, many (perhaps most?) native speakers with add 个子 as a Topic. Often the Topic seems to limit/clarify the scope of the comment in sentences that are open to multiple interpretations. (For example, 她年纪很大, lit. "She is great in age.") My sense is 我的同学很高 completely unambiguous. I think 个子 is added merely for rhythm.
I am a native speaker from HK and I think that it's acceptable to add 个子 in the sentence . however , the official translation should be 我的同学很高