The premise is interesting, but the execution leaves a lot to be desired.

Basically, Julia witnesses her classmates vanishing from her sight in the middle of class. So she decides to drill down and get to the bottom of what's going on.

An unrealistic premise is perfectly fine, of course, as lots of fun books (i.e. My Teacher is an Alien, Pyrates, How a Weirdo and a Ghost Can Change Your Entire Life) hinge on them and manage to make their story a lot of fun. It's how the premise is handled that changes everything.

For one, a good deal of skepticism would help tremendously and better ground the story in reality. In My Teacher is an Alien, Susan convinced Peter to come with her, as he didn't believe her story. In How a Weirdo and a Ghost Can Change Your Entire Life, Teddy's attempts to convince Martha that he really could communicate with ghosts only made her angry at him until she eventually tried it out herself out of boredom.

Here, Julia tells her friends Lori and Jeff about what happened, and while they think "huh, I don't remember disappearing", they agree to help her solve the mystery and even get excited about it. Though Julia later manages to find more definite proof that something strange happened, they believe her from the start. I think they should have only believed her after she gave them the more definite proof, and even then remained skeptical.

Next up, we have convenient coincidences for the sake of plot. Next door to the school is a military base. I won't go into details about how Julia sneaks in, but let's say military security kinda sucks in fictionland.

Things get a lot more interesting once she's inside and discovers the cause of all the troubles. In fact, there's even a really interesting and creative scene that I won't spoil. The story doesn't get any more plausible there, but it lost its believability a long time ago anyway.

It's actually a fun story if you turn off your brain. But I do think the age group it's aimed at should be lowered slightly, and the book renamed "The Day the Third (or Fourth) Grade Disappeared". The story wouldn't be any more realistic, but if aimed at younger kids, the sheer amount of implausibility would be easier to overlook. Real fifth graders would easily recognize multiple reasons why the story just wouldn't work, fun as it is when it really gets going.