An 8th Grader's times must be looked at based upon the type of training he/she is doing. Sure, it's possible to run sub-5 if an 8th Grade boy is training like a high schooler. But what if he isn't training at all? What if he just runs a mile to fulfill a PE requirement?
Also, the level of physical maturity in 8th Graders plays a significant role in what their times will be.
I ran 5:42 in 8th Grade, then ran 4:42 as a 9th Grader (and 10:00 for 2 miles).
All in all, I'd say what's "good" as far as times go for 8th Graders encompasses a very wide, very speculative range.