Why do HS runners always positive split 5k races but negative split in shorter races??

In which universe are you living such that average high schoolers are regularly closing hot paces in 59?

Assuming you are referring to elite fields of HSers, a lot of high level races close with a negative split final lap in general. In high school I am sure this may be exacerbated by relative inexperience by and large. I was at a meet Saturday in which the field opened in a 66, top runner closed in ~60 like you are referencing, for a 4:17 PR. This could also be the result of the fact that it is still early season and most top runners are flexing their talons rather than going wire to wire dragging themselves to shave a second or two from more optimal pacing. After all, come state, being able to win a slightly tactical race with a big kick will be more helpful.

In most HS races, the kids actually go out far too fast. At the same meet I saw two kids separately in the 4x8 and open 8 go 61-79 to run 2:20. The second-fastest heat of the 16 opened in a 66 before dumping most of the field into the low 4:40s while the fast heat opened in the same pace. Kids are crazy, still gaining experience, and a myriad of tactical wonders are going to come about. Just my thoughts.

Also, XC races are far more difficult to standardize paces for (since courses vary) and are gutsy battles for wins more often than controlled time trials. I think this results in positive splitting because of aggressive packs going out harder than they are able to handle and forcing the leaders to quicken their pace or else risk losing contact. And the same rule of inexperienced HSers doing silly things applies.

High schoolers don't negative split anything. That's a skill that comes with maturity. Many people never learn.