ok. no, you shouldn't be feeling that way. academic decisions should be left to the academic staff. (imo)
but that said, i'm only saying that because i honestly wouldn't give a monkeys lol.
none of us can tell you how you should be feeling, but you have to work out why you are feeling that way and deal with it.
i honestly think if you approach the teacher and question her professional judgement because you don't like tests it might come out the wrong way...
fwiw i have one statemented who is also very able lol, and one who has been twice assessed by externals in the same manner. (with no-one asking my permission )
in the words of catherine tate, am i bovvered? no, but obviously you are. parents are as different as the kids they spawn lol, and teachers have enough to do second-guessing the class without having to worry about whether mrs x will like their assessment techniques. you trust her judgement, and she takes your advice on how to handle his sn, so why are you not extending her the same courtesy in matters where she is the expert?
do speak to her, but honestly, take a deep breath first and just explain you don't like assessments.
on a slightly different note, how are you expecting them to differentiate his work appropriately if they are not allowed to work out what level he is currently at? or is this not a priority for you? (that's a serious question btw - it would presumably be very time-consuming for them to do it any other way? i assume he doesn't have 1-1 as obviously this would have resolved the supervision issue)
no adversarial tone intended - genuinely interested.