When an 18-year-old female accidentally leaves a love letter to her female crush on the kitchen counter, she must find a way to come out to her parents before her mother accuses her father of cheating.

I think the term, 'unsuspecting parents' works well.

Not sure if you need to tell us she is confessing her feelings since you already said it was a love letter, so it is implied.

If you dropped that line but kept everything else the same it would read like this.
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"When an 18-year-old closeted lesbian accidentally leaves a love letter out in the open, she must bite the bullet and come out to her unsuspecting parents before her mother accuses her father of cheating."

This will help a bit with the word count.

Anyway, I like the concept, and as I said earlier, the sense of what the story is about comes through in the logline.

would something like that work better?

Thanks for the feedback.

Would something like "When an 18 year old closeted lesbian accidentally leaves a love letter out in the open, confessing her feelings to one of her classmates, she must bite the bullet and come out to her unsuspecting parents, before her mother accuses her father of cheating."