If you are determined to raise a specific character's Affection ASAP, you might have to make some OOC dialogue choices or other decisions during the barbecue event. For example, Elio likes a competitive MC, but it would be OOC for me to pick that for the MC I created, so I had to give up on those Affection points.
You also might have to make an effort to not be as appealing to the other characters if you want Percy to have the highest points. If your dialogue and actions are very appealing to Elio and Jamie, but less appealing to Percy, that might be the problem.
In the full game, this won't be as big of an issue, because you'll have multiple semesters to build Affection with a specific character, and you're bound to find Events and dialogue choices that work for you. The relationship might build more slowly, but you'll still get there!