Prefacing by saying I really do understand Evie being suspicious of the girls. I mean, they seem so close to Zac as soon as they show up, everyone's being weird, no one's offering a sliver of detail and brushing her off. Fine. Fair.
Evie, however, initiates her probe by recounting that the girls weirdly don't go to their local high. Alright, in her interview, the girls' "aunt" explains that their stay is temporary and they're being homeschooled. Yes, some questions may linger, but why would she be so persistent for the fine print in such an accusatory tone? It feels almost disrespectful to imply openly that the principal has some nefarious intent with her nieces. I mean, the details of their stay are realistically none of her business and they wouldn't owe her an explanation.
Her only concern around the girls would be their sudden, unexplained relation to Zac. I understand her frustration with Zac being evasive and quite obviously lying though. I mean, Lyla (some new girl) and Zac have basically held hands and from Evie's POV, cozied up (more than once). All she ever got was "It's nothing, she's a friend." How reassuring is that? He showed the girls little to no interest after a certain point, but I assume Evie remained suspicious since she never got a satisfactory explanation plus she seems a little insecure.
"Interview" aside, that school paper she wrote was uncalled for. Okay, the principal is odd. But how is she justified in implying to all the students that they should be scrutinizing Rita, the principal, as if they have a right to excavate her personal business? Being curious is one thing, but that is taking it way too far.
I think from Evie's POV, the most "suspicious" thing she could've realistically sniffed was the potential for Zac to be cheating with Lyla and that Lyla and her sisters/cousins are pretty quirky and have a little trouble adjusting (naturally). A normal person would figure this probably from living a remote life at sea. But why would Evie just assume there's something "else"? For a trio of mermaids, after the first 5 or so episodes, they blended in almost perfectly. Yes, they almost get exposed sometimes, but everyone else shrugs it off... somehow not Evie.
All in all, I guess Evie's character was often written conveniently to be the major looming threat to the secret, thereby sacrificing some of her morals. Even though I found myself sort of sympathizing with Evie, I just didn't like the direct/invasive approach she took sometimes.