Comparisons Doubt

It depends very much on what the sentence is trying to say, but the corrected version means something very strange. If you consider this correct sentence:

The farmer's vegetables are unlike those at the supermarket.

then the word "those" stands in for "vegetables" so this sentence has the same meaning:

The farmer's vegetables are unlike vegetables at the supermarket.

The word "those" does not stand in for the whole phrase "the farmer's vegetables"; if it did, we'd get the nonsensical meaning

The farmer's vegetables are unlike the farmer's vegetables at the supermarket.

Simplifying your original corrected sentence, it says

Courtney's experiences at Haleford University were unlike those at high school.

That has this meaning:

Courtney's experiences at Haleford University were unlike experiences at high school.

There's nothing grammatically wrong here, but the sentence says something I would guess is unintended -- it says Courtney's university experiences were different from general (whose?) experiences at high school. If the sentence is trying to say what I expect it's trying to say -- that is, if the sentence is trying to compare Courtney's university experience with her high school experience, then the corrected version is wrong. The sentence would need to say

Courtney's experiences at Haleford University were unlike her experiences at high school.

or you could replace "her experiences" with the simple possessive "hers", though that feels a bit unclear to me. If you wanted to use "those", you could say "Courtney's experiences at Haleford were unlike those she had in high school", or something similar, but there needs to be some extra wording to indicate that "those" describes Courtney's experiences, and not just experiences anyone might have.