As said, Conditions for applying are given here:
https://developer.apple.com/wwdc21/swift-student-challenge/
Applying: Build your Swift playground, answer a few written prompts, provide documentation, and submit.To be eligible for the Challenge, you must:Be 13 years of age or older, or the equivalent minimum age in the relevant jurisdiction (for example, 16 years of age in the European Union);Be registered for free with Apple as an Apple developer or be a member of the Apple Developer Program; andFulfill one of the following requirements:Be enrolled in an accredited academic institution or official homeschool equivalent;Be enrolled in a STEM organization’s educational curriculum;Be enrolled in an Apple Developer Academy; orHave graduated from high school or equivalent within the past 6 months and be awaiting acceptance or have received acceptance to an accredited academic institution.
See last condition:
or Have graduated from high school or equivalent within the past 6 months and be awaiting acceptance or have received acceptance to an accredited academic institution.
So, july 2020 graduation does not match, but I do hope you could enroll as you might been enrolled in an accredited academic institution and above 13