Ryun's 5:38 to 3:55 is probably the best out there. However, Hobbs Kessler probably has a pretty similar start point, as I believe he ended his freshman year with a 4:59 PR and of course ended up going sub 4 and 3:34 as a senior.
Also for longer distance Chris Derrick was mentioned earlier but he's worth mentioning again. He started out with a mid 18 minute 5k PR and was an also ran on the Nequa Valley frosh squad. Coach Vandersteen brought him along and he won NTN, both individually and his team won, plus ran 13:55 at Arcadia his senior year. I think I heard he was basically Nequa's top runner his sophomore year after getting serious training over summer. He's commented on how he was meant to be a regular white collar guy/CPA but Vandersteen had other ideas! Had to mention hims again because CD is one of my favorite runners!