When she was in college, she used to think she was better than those who got lower grades than her / got lower grades than she did / got lower grades than she got.
Are the three above phrases in bold possible in the context of the example?
When she was in college, she used to think she was better than those who got lower grades than her / got lower grades than she did / got lower grades than she got.
Are the three above phrases in bold possible in the context of the example?
They got lower grades than her.
They got lower grades than she did.
They got lower grades than she got.
All the sentences are correct grammatically.
Although, the use of the accusative form (as her in the first sentence) is common, people think the nominative form (she) is more refined and formal English. However, we usually use auxiliaries/helping verbs after the nominative forms of pronouns to avoid repetition of a verb mentioned earlier.
So we should say "she did" instead of she got in the last sentence.