Following a high school reunion a man seeks forgiveness from a childhood sweetheart who he wronged 15 years prior. She however has plans for revenge and will stop at almost nothing to ruin his perfect life.

If I understood this correctly, I'm intrigued.
I'm assuming she had put it behind her in one way or another (ie. that she buried the trauma in her subconscious; or even that she assumed, herself, that she had forgiven him and had moved on) until the feelings resurface upon seeing him.
There's a lot of potential there.

Formatting-wise, don't forget your commas.

"Following a high school reunion, a man seeks forgiveness from a childhood sweetheart who he wronged fifteen years prior. She, however, has plans for revenge, and will stop at almost nothing to ruin his perfect life."

Punctuation doesn't add to the word count, and it helps with the ebb and flow of the read.

Even with the commas, it still doesn't read as smoothly as it could; and unfortunately I don't think it's a matter of dropping a word or two in order to achieve that rhythmic, poetic flow that makes a logline beautiful.
Mind if I try?

"Faced with a blast from his past at his high school reunion, an opportunistic man seeks forgiveness from his former girlfriend, but inadvertently sparks a revenge plot in her that could ruin him." (33 words)
This is if the man is the MC.

If the woman is the MC, I could see it reading something like this:
"After a brief encounter at their high school reunion, a woman spurned releases a level of hell's fury with the aim to ruin her ex-boyfriend and everything he holds dear." (30 words)

Truthfully, I don't think either of my versions remain close enough to the original spirit of your logline, but hopefully (maybe?) you get a sense of rhythm and flow from them that you can try to emulate, should you attempt another draft?

At any rate, kudos for the story concept.
I now see this was posted in 2014. I hope you were able to do something great with the script.

Since the woman is the lead, my first question is. How was she wronged?

Yes, a movie can have two protagonists. But in this case it seems as though there is a protagonist and and antagonist. (Since they are opposed to one another)

that certainly solves the switch between protagonist...

i was planning on the woman being the lead after the set up from the man's background, but maybe too much focus on the man in the logline?

Can't a story have two protagonists? (genuine question, sorry if it sounds sarcastic)

After a high school reunion, a man seeks forgiveness for his past, but instead must deal with vengeance.

You switch protagonists halfway through the logline.

Who is the lead character?

i think you're spot on. single sentence it is. thanks so much. just the interesting part now....