That would be fine. Quite a few SA appointees hold a place in a civilian school until I/A/R Day. Some hold a place even longer, in case anything happens during summer training.
From your perspective, the main issue is whether you’re willing to lose your deposit (usually a few hundred dollars). For us, that was a small premium to pay for an “insurance policy” on an SA. You may lose it by going to an SA, but you’ll recoup it in spades with the “free” education.
From the school’s perspective, they care mainly about that deposit. You won’t hurt any feelings or burn any bridges by not showing up in the fall. In fact, schools plan for this attrition. They even have a name for it: “summer melt.” That said, it doesn’t hurt to send an email prior to the school year, saying “thanks but no thanks.”