Its not like she just had a HS diploma trying to get into Harvard. She had a Bachelors degree, good grades and was in a number of extracurricular activities. Its not a stretch to get a BA or BS and then go on to grad school in a completely different field. The movie makes it out like she was in way over her head and wouldn't be able to handle it.
shareWell duh.
We saw her study hard for a little while and ace the law school entrance exam, beating kids who'd taken years of pre-law, that was our first clue.
I don't see it that way.
Early in the film, where she is shopping for a dress a sales assistant assumes that because she is blonde, pretty and into fashion she must be a bimbo. However, Elle sees straight through her lies, and is not conned into buying something at an overpriced rate. This was meant to demonstrate that despite her appearance, Elle wasn't anybody's fool and could argue her case when she needed to.
When she managed to get into Harvard, everybody else made the same mistake as the sales assistant earlier. Because she turned up dressed in pink, carrying her little dog and just didn't look like a stereotypical law student they assumed she was in over her head. Something that by the end of the movie was proven to be incorrect. She was made valedictorian.
Elle might not have originally put her intelligence to good use when all she cared about was Warner marrying her. But that doesn't mean it wasn't there when she actually applied it.