At the rink. Again.
But that's not true - a skater who passes the US Senior MIF and FS tests can no longer compete as a Junior at US quaifying competitions (Regionals, Sectionals, Nationals). There are select situations for nonqualifying competitions where skaters are allowed to "skate down" in the same way that skaters can "skate up" a level. Those situations are for skaters who have passed their Senior FS but are competing on the JGP circuit.
Because of the disparity in the way skaters can skate Senior Nationals within various countries, it's within the purview of each NGB to determine who will represent the country at both Senior and Junior Worlds (and 4CC/Europeans). The main mission of US Figure Skating is to create World and Olympic Champions. In order to do that, they need as many spots as possible in the JGP and at JWs to get exposure for the atheletes who are promising but aren't quite ready for prime time (Senior GP and Worlds) yet. To that end, it's in US Figure Skating's best interest to send Agnes Zawadzki, Christina Gao and Keegan Messing to JWs.
If all things were equal and all countries had similar test criteria, then perhaps you could preclude those who have passed the International Senior FS test from skating at Junior Worlds. If you think all countries could come to a consensus on what should/shouldn't be included on that test, I have some swamp land in Arizona for sale....