Former high school Ms. Popular, Sophie, is now the mother of an autistic child. Her life's ambitions are now different and she is burdened by a misplaced sense of regret. Tonight she is on her way to her high school reunion to face her former classmates. The night turns out to be a difficult one, where her moral standards are challenged.

You have all the elements in here that are needed, and then some more. I think it will be best if you take out the lines “her’s life’s ambitions are now different…regret” and “night turns out to be difficult… are challenged.” You can use some of that in a single word if you want to. For instance:
Former high school Ms. Popular, Sophie who’s now a mother of an autistic child is on her way to her high school reunion to face her former classmates only to find a difficult night of moral judgements up ahead.

This is over the top of my head so I’m sure you can come up with something better.
The concept is good, but this is highly execution dependent so the one page synopsis is as important as the logline. I’d start working on that as well.
Good luck.