Why does Laura Harris put on such a horribleDixie accent that even a New Yorker would find painfully fake? Because her character is an alien, and is thus not a native speaker of the accent! Indeed, in the original version of the script, she drops her accent just before The Reveal.
Delilah's Clark Kenting to avoid being detected by the aliens should never have worked... and it didn't. In the scene at Zeke's house not long after, she's revealed to have been infected. Her Clark Kenting worked about as well as it would have in Real Life.
One of Zeke's money-making schemes other than dealing drugs is selling VHS tapes with nude scenes featuring actresses like Jennifer Love Hewitt and Neve Campbell. Only those two never did nude scenesnote At least, not at the time. While Hewitt has, to paraphrase another Kevin Williamson film, "kept it PG-13", Campbell has done nude scenes in a number of films since. There was Campbell's lesbian makeout scene with Denise Richards in Wild Things, but unlike Richards, she didn't actually show anything in that scene.... and Zeke probably knew it. Given that the guy's day job is fairly sleazy, it wouldn't be at all surprising if he were ripping off those two horny classmates.
Marybeth tells everyone she's from Atlanta. She's not only from another planet, she's from a water planet. Kind of like... Atlantis.
About midway through the film, two of Zeke's longtime customers come up to him and start demanding his drugs. There's not one, but two explanations for their behavior that make sense: either those two guys figured out on their own that Zeke's drugs can protect them from the aliens, or they've been puppetized themselves and are trying to stop him.
Considering that the movie takes great inspiration from The Breakfast Club, one could potentially figure out the big twist at the end. In both movies, a small group of students, each embodying a specific teenage stereotype, receives the main focus. Just like in The Breakfast Club, you have the brain (Casey), the athlete (Stan), the basket case (Stokely), the princess (Delilah), and the criminal (Zeke), but there's someone missing: Marybeth (the new girl). Seeing how she doesn't belong in the group, you can guess that she may not be all that she seems.
After Nurse Harper is infected, the entire left side of her head is covered in blood. When she steps out of the teacher's lounge less than a minute later, it's all gone. Ditto for the police officer moments later (though we don't see his infection). How did they get themselves cleaned off so quickly?
The parasite is said to dry out its hosts, so that they need regular water to keep hydrated. So maybe the blood dried up that fast.
For that matter, Nurse Harper's infection with the parasite is depicted as being incredibly painful and bloody. And when Principal Drake infects one of the police officers, we hear a noise from inside her office. When the faculty gets around to infecting the students, surely someone in the huge lines outside the nurse's office would've heard something, or seen a few drops of blood that they may have missed when they were being cleaned off? And how did the blood not get on any of their clothes?
Once the parasites are on someone they can burrow in. Having an orifice like an ear just makes the process easier and less noticeable. Maybe they are forced to ingest them, leaving only a little blood inside their mouths.
And maybe they learned once Casey and Delilah saw Nurse Harper's attack. They realised they'd need a quieter way of infecting the students.